Blockchain Papers

Follow blockchain research across journals, conferences, and preprint repositories.

2,964 papersLast indexed Aug 31, 2026
Search papers

Paper index

2,964 results · page 106 of 124

Clear filters
Jun 14, 2019·Research in International Business and Finance
86 cites
Lead-Lag relationship between Bitcoin and Ethereum: Evidence from hourly and daily data

Imtiaz Sifat, Azhar Mohamad, Mohammad Syazwan Bin Mohamed Shariff

This paper investigates lead-lag relationship between heavyweight cryptocurrencies Bitcoin and Ethereum. Traditional studies of information flow between markets preponderate on cash vs. futures, whereby researchers are interested in the stabilizing impact of futures on spot markets. While interest in the same relationship in the nascent cryptocurrency sphere is emerging, little is known regarding price leadership between these assets. In this paper, we employ a battery of statistical tests—VECM, Granger Causality , ARMA, ARDL and Wavelet Coherence—to identify price leadership between the two crypto heavyweights Bitcoin and Ethereum. Based on one year hourly and daily data from August 2017 through to September 2018, our tests yield varied results but largely suggest bi-directional causality between the two assets. Moreover, the results indicate that intraday crypto traders can barely exploit Bitcoin-Ethereum hourly or daily price discovery process to their advantage.

Open access
2 source records
Blockchain Technology Applications and Security
Market Dynamics and Volatility
Complex Systems and Time Series Analysis
Original source
Jun 13, 2019·Royal Society Open Science
42 cites
Information-theoretic measures for nonlinear causality detection: application to social media sentiment and cryptocurrency prices

Z. Keskin, Tomaso Aste

Information transfer between time series is calculated using the asymmetric information-theoretic measure known as transfer entropy. Geweke’s autoregressive formulation of Granger causality is used to compute linear transfer entropy, and Schreiber’s general, non-parametric, information-theoretic formulation is used to quantify nonlinear transfer entropy. We first validate these measures against synthetic data. Then we apply these measures to detect statistical causality between social sentiment changes and cryptocurrency returns. We validate results by performing permutation tests by shuffling the time series, and calculate the Z -score. We also investigate different approaches for partitioning in non-parametric density estimation which can improve the significance. Using these techniques on sentiment and price data over a 48-month period to August 2018, for four major cryptocurrencies, namely bitcoin (BTC), ripple (XRP), litecoin (LTC) and ethereum (ETH), we detect significant information transfer, on hourly timescales, with greater net information transfer from sentiment to price for XRP and LTC, and instead from price to sentiment for BTC and ETH. We report the scale of nonlinear statistical causality to be an order of magnitude larger than the linear case.

Open access
3 source records
Market Dynamics and Volatility
Complex Systems and Time Series Analysis
Stock Market Forecasting Methods
Original source
Jun 8, 2019·Finance research letters
52 cites
The effects of the introduction of Bitcoin futures on the volatility of Bitcoin returns

Wonse Kim, Junseok Lee, Kyungwon Kang

This paper investigates the effects of the launch of Bitcoin futures on the intraday volatility of Bitcoin. Based on one-minute price data collected from four cryptocurrency exchanges, we first examine the change in realized volatility after the introduction of Bitcoin futures to investigate their aggregate effects on the intraday volatility of Bitcoin. We then analyze the effects in more detail utilizing the discrete Fourier transform. We show that although the Bitcoin market became more volatile immediately after the introduction of Bitcoin futures, over time it has become more stable than it was before the introduction.

Open access
3 source records
Blockchain Technology Applications and Security
Complex Systems and Time Series Analysis
Market Dynamics and Volatility
Original source
Jun 3, 2019·arXiv (Cornell University)
10 cites
C2P2: A Collective Cryptocurrency Up/Down Price Prediction Engine

Chongyang Bai, Tommy E. White, Linda Xiao, V. S. Subrahmanian · 5 authors

We study the problem of predicting whether the price of the 21 most popular cryptocurrencies (according to coinmarketcap.com) will go up or down on day d, using data up to day d-1. Our C2P2 algorithm is the first algorithm to consider the fact that the price of a cryptocurrency c might depend not only on historical prices, sentiments, global stock indices, but also on the prices and predicted prices of other cryptocurrencies. C2P2 therefore does not predict cryptocurrency prices one coin at a time --- rather it uses similarity metrics in conjunction with collective classification to compare multiple cryptocurrency features to jointly predict the cryptocurrency prices for all 21 coins considered. We show that our C2P2 algorithm beats out a recent competing 2017 paper by margins varying from 5.1-83% and another Bitcoin-specific prediction paper from 2018 by 16%. In both cases, C2P2 is the winner on all cryptocurrencies considered. Moreover, we experimentally show that the use of similarity metrics within our C2P2 algorithm leads to a direct improvement for 20 out of 21 cryptocurrencies ranging from 0.4% to 17.8%. Without the similarity component, C2P2 still beats competitors on 20 out of 21 cryptocurrencies considered. We show that all these results are statistically significant via a Student's t-test with p<1e-5. Check our demo at https://www.cs.dartmouth.edu/dsail/demos/c2p2

Open access
3 source records
Blockchain Technology Applications and Security
Stock Market Forecasting Methods
Market Dynamics and Volatility
Original source
Jun 1, 2019·Management of Organizations Systematic Research
2 cites
Possible Impact of Facebook’s Libra on Volatility of Bitcoin: Evidence from Initial Coin Offer Funding Data

Chamil W. Senarathne

Abstract This paper examines the impact of Libra on volatility of Bitcoin using the classical framework of C. G. Lamoureux and W. D. Lastrapes (1990). ARCH and GARCH effects disappear when lagged ICO funding size is included in the variance equation. A negative association between volatility and funding size and the disappearance of volatility persistence (long-term volatility effect) suggest that Libra, as a dominant new currency, is likely to stabilize the cryptocurrency market and enhance potential for currency diversification. Furthermore, it is revealed that the stability cannot be ensured merely by backing decentralized blockchain instruments, such as Bitcoin, with bank deposits, government securities or exchange rate.

Open access
2 source records
Blockchain Technology Applications and Security
Market Dynamics and Volatility
Complex Systems and Time Series Analysis
Original source
May 31, 2019·Journal of risk and financial management
7 cites
Is Bitcoin a Relevant Predictor of Standard & Poor’s 500?

Camilla Muglia, Luca Santabarbara, Stefano Grassi

The paper investigates whether Bitcoin is a good predictor of the Standard & Poor’s 500 Index. To answer this question we compare alternative models using a point and density forecast relying on Dynamic Model Averaging (DMA) and Dynamic Model Selection (DMS). According to our results, Bitcoin does not show any direct impact on the predictability of Standard & Poor’s 500 for the considered sample.

Open access
Market Dynamics and Volatility
Monetary Policy and Economic Impact
Complex Systems and Time Series Analysis
Original source
May 29, 2019·Fiscaoeconomia
13 cites
The Volatility Structure of Cryptocurrencies: The Comparison of GARCH Models

İbrahim Korkmaz Kahraman, Habib Küçükşahin, Emin ÇAĞLAK

Forecasting models based on the assumption that returns are normally distributed do not perform sufficiently on shallow markets. These models are more likely to fail in the estimation of the extreme points that can be reached especially at high volatility markets, and this situation is led to investors in predicting volatility. In the volatility forecasting of crypto money, which is seen as an alternative investment tool for the financial investors, single volatility models such as, ARCH, GARCH, T-GARCH, GARCH-M, E-GARCH, and I-GARCH and long memory models (AP-GARCH and C-GARCH) was utilized. In addition, the most suitable model was tried to be tested among the models used for volatility estimation. In this context, the price data of Bitcoin, Ethereum and Ripple cryptocurrency with the highest market value in the crypto money market have been utilized between 24/08/2016-07/05/2018. According to the results of the research, for Bitcoin and Ethereum, the volatility effect of the shocks is permanent and the effect of the positive shocks is more than that of the negative shocks, whereas for Ripple, the volatility effect of the shocks is transient and the passivity of the volatility is short.

Open access
Market Dynamics and Volatility
Financial Risk and Volatility Modeling
Blockchain Technology Applications and Security
Original source
May 29, 2019·Journal of Business Economics and Management
43 cites
INTER-MARKETS VOLATILITY SPILLOVER IN U.S. BITCOIN AND FINANCIAL MARKETS

Muhammad Owais Qarni, Saqib Gulzar, Syeda Tamkeen Fatima, Majid Jamal Khan · 5 authors

This paper investigates the volatility spillover dynamics between U.S. Bitcoin and financial markets from July 19, 2010 to December 29, 2017. Diebold and Yilmaz (2012) volatility spillover index, Barunik, Kocenda, and Vacha (2017) Spillover Asymmetry Measure, and Barunik and Krehlik (2018) frequency connectedness methodologies are applied to investigate the time varying dynamics of volatility spillover among U.S. Bitcoin and financial markets. The findings of the study indicate the presence of low level of integration and contagion between U.S. Bitcoin and financial markets. Asymmetric nature of volatility spillover is also detected. The connectedness among the U.S. Bitcoin and financial markets is found to be concentrated at high frequency, suggesting that markets process information rapidly. Moreover, the turbulence in Bitcoin market will have insignificant effect on U.S. financial markets. This non-contagion nature of Bitcoin markets provides significant risk hedging and diversification benefits for domestic and foreign investors in the U.S.

Open access
Market Dynamics and Volatility
Blockchain Technology Applications and Security
Complex Systems and Time Series Analysis
Original source
May 23, 2019·Physica A Statistical Mechanics and its Applications
78 cites
Real-time prediction of Bitcoin bubble crashes

Min Shu, Wei Zhu

In the past decade, Bitcoin as an emerging asset class has gained widespread public attention because of their extraordinary returns in phases of extreme price growth and their unpredictable massive crashes. We apply the log-periodic power law singularity (LPPLS) confidence indicator as a diagnostic tool for identifying bubbles using the daily data on Bitcoin price in the past two years. We find that the LPPLS confidence indicator based on the daily Bitcoin price data fails to provide effective warnings for detecting the bubbles when the Bitcoin price suffers from a large fluctuation in a short time, especially for positive bubbles. In order to diagnose the existence of bubbles and accurately predict the bubble crashes in the cryptocurrency market, this study proposes an adaptive multilevel time series detection methodology based on the LPPLS model and finer (than daily) timescale for the Bitcoin price data. We adopt two levels of time series, 1 hour and 30 minutes, to demonstrate the adaptive multilevel time series detection methodology. The results show that the LPPLS confidence indicator based on this new method is an outstanding instrument to effectively detect the bubbles and accurately forecast the bubble crashes, even if a bubble exists in a short time. In addition, we discover that the short-term LPPLS confidence indicator highly sensitive to the extreme fluctuations of Bitcoin price can provide some useful insights into the bubble status on a shorter time scale - on a day to week scale, and the long-term LPPLS confidence indicator has a stable performance in terms of effectively monitoring the bubble status on a longer time scale - on a week to month scale. The adaptive multilevel time series detection methodology can provide real-time detection of bubbles and advanced forecast of crashes to warn of the imminent risk.

Open access
2 source records
Complex Systems and Time Series Analysis
Blockchain Technology Applications and Security
Market Dynamics and Volatility
Original source
May 23, 2019·Small Business Economics
78 cites
Initial coin offerings (ICOs): market cycles and relationship with bitcoin and ether

Christian Masiak, Joern Block, Tobias Masiak, Matthias Neuenkirch · 5 authors

Abstract We apply a vector autoregression (VAR) model to investigate the market cycles of Initial Coin Offerings (ICOs) as well as their relationships with bitcoin and ether. Our sample covers 104 weekly observations between January 2017 and December 2018. Our results show that ICO market cycles exist and that shocks to the growth rates of ICO volumes are persistent. In addition, shocks in cryptocurrency returns have a substantial and positive effect on ICO volumes. In contrast, the volatility of cryptocurrency returns does not significantly affect ICO volumes. Our results are robust to using (i) the number of successfully completed ICO campaigns instead of ICO volumes and (ii) ICO data from a different data source. Our study has implications for financial practice, in particular for cryptocurrency investors and entrepreneurial firms conducting ICOs.

Open access
Blockchain Technology Applications and Security
FinTech, Crowdfunding, Digital Finance
Market Dynamics and Volatility
Original source
May 23, 2019·Research in International Business and Finance
106 cites
Do investors herd in cryptocurrencies – and why?

Vasileios Kallinterakis, Ying Wang

No abstract is available for this record.

Open access
Blockchain Technology Applications and Security
Market Dynamics and Volatility
Complex Systems and Time Series Analysis
Original source
May 21, 2019·Physica A Statistical Mechanics and its Applications
144 cites
An approach to predict and forecast the price of constituents and index of cryptocurrency using machine learning

Reaz A. Chowdhury, M. Arifur Rahman, M. Sohel Rahman, M. R. C. Mahdy

At present, cryptocurrencies have become a global phenomenon in financial sectors as it is one of the most traded financial instruments worldwide. Cryptocurrency is not only one of the most complicated and abstruse fields among financial instruments, but it is also deemed as a perplexing problem in finance due to its high volatility. This paper makes an attempt to apply machine learning techniques on the index and constituents of cryptocurrency with a goal to predict and forecast prices thereof. In particular, the purpose of this paper is to predict and forecast the close (closing) price of the cryptocurrency index 30 and nine constituents of cryptocurrencies using machine learning algorithms and models so that, it becomes easier for people to trade these currencies. We have used several machine learning techniques and algorithms and compared the models with each other to get the best output. We believe that our work will help reduce the challenges and difficulties faced by people, who invest in cryptocurrencies. Moreover, the obtained results can play a major role in cryptocurrency portfolio management and in observing the fluctuations in the prices of constituents of cryptocurrency market. We have also compared our approach with similar state of the art works from the literature, where machine learning approaches are considered for predicting and forecasting the prices of these currencies. In the sequel, we have found that our best approach presents better and competitive results than the best works from the literature thereby advancing the state of the art. Using such prediction and forecasting methods, people can easily understand the trend and it would be even easier for them to trade in a difficult and challenging financial instrument like cryptocurrency.

Open access
2 source records
Blockchain Technology Applications and Security
Market Dynamics and Volatility
Complex Systems and Time Series Analysis
Original source
May 15, 2019·HAL (Le Centre pour la Communication Scientifique Directe)
1 cites
How do futures contracts affect Bitcoin prices ?

Jamal Bouoiyour, Refk Selmi

Bitcoin futures were launched by the Chicago Board of Options Exchange and the Chicago Mercantile Exchange group on December 18th, 2017. This study stands as a first attempt to explore the reactions of Bitcoin spot market to the launch of futures contracts. Using an event-study methodology and an adjusted asset pricing model, we show that Futures trading drove up the price of Bitcoin immediately after the announcement day. This reaction started to decrease noticeably following the launch of the futures contracts. Such outcome seems in line with the trading behavior that typically accompanies the launch of futures markets for an asset.

Open access
Blockchain Technology Applications and Security
Financial Markets and Investment Strategies
Market Dynamics and Volatility
Original source
May 14, 2019·Applied Economics and Finance
2 cites
A Simple Approach to Assess if a Financial “Bubble” is Present: The Case of Bitcoin

Vítor Manuel Araújo da Fonseca, Manuel A. R. da Fonseca

This article’s goal is to evaluate if the recent price behavior of Bitcoin can be characterized as a financial market “bubble”. To deal with this assessment, we adopt a statistical definition of a “bubble” derived from the efficient market hypothesis and we propose a simple method to test this proposition, based on the time-series model known as random walk. We analyze the data available for Bitcoin prices, together with an asset selected as benchmark, and perform statistical tests derived from simple regression equations. The main conclusion is that there is consistent evidence that that Bitcoin follows the pattern of a financial “bubble” – at least, such pattern is more evident in the case of Bitcoin than in the stock index used as benchmark.

Open access
Complex Systems and Time Series Analysis
Blockchain Technology Applications and Security
Market Dynamics and Volatility
Original source
May 13, 2019·The Annals of Applied Statistics
31 cites
Asymmetric tail dependence modeling, with application to cryptocurrency market data

Yan Gong, Raphaël Huser

Since the inception of Bitcoin in 2008, cryptocurrencies have played an increasing role in the world of e-commerce, but the recent turbulence in the cryptocurrency market in 2018 has raised some concerns about their stability and associated risks. For investors, it is crucial to uncover the dependence relationships between cryptocurrencies for a more resilient portfolio diversification. Moreover, the stochastic behavior in both tails is important, as long positions are sensitive to a decrease in prices (lower tail), while short positions are sensitive to an increase in prices (upper tail). In order to assess both risk types, we develop in this paper a flexible copula model which is able to distinctively capture asymptotic dependence or independence in its lower and upper tails simultaneously. Our proposed model is parsimonious and smoothly bridges (in each tail) both extremal dependence classes in the interior of the parameter space. Inference is performed using a full or censored likelihood approach, and we investigate by simulation the estimators' efficiency under three different censoring schemes which reduce the impact of non-extreme observations. We also develop a local likelihood approach to capture the temporal dynamics of extremal dependence among two leading cryptocurrencies. We here apply our model to historical closing prices of five leading cryotocurrencies, which share most of the cryptocurrency market capitalizations. The results show that our proposed copula model outperforms alternative copula models and that the lower tail dependence level between most pairs of leading cryptocurrencies -- and in particular Bitcoin and Ethereum -- has become stronger over time, smoothly transitioning from an asymptotic independence regime to an asymptotic dependence regime in recent years, whilst the upper tail has been relatively more stable overall at a weaker dependence level.

Open access
3 source records
Financial Risk and Volatility Modeling
Complex Systems and Time Series Analysis
Market Dynamics and Volatility
Original source
May 6, 2019·International Journal of Financial Research
4 cites
Effect of Weather on Cryptocurrency Index: Evidences From Coinbase Index

Chinnadurai Kathiravan, Murugesan Selvam, Balasundram Maniam, Sankaran Venkateswar · 6 authors

This study proposes to investigate the dynamic relationships between the three weather factors (temperature, humidity, and wind speed) in New York City of USA and Coinbase Index from Federal Reserve Bank of St. Louis, in the USA. Statistical tools like Descriptive Statistics, Unit Root, Granger Causality Test and Johansen Co-Integration test were employed. This study clearly found that the temperature influenced the investors’ mood and their investment decision in respect of Cryptocurrency index (Coinbase Index) and also found that there was long run equilibrium between the sample variables during the study period. The results of study provided strong evidence against the Efficient Market Hypothesis (EMH).

Open access
Market Dynamics and Volatility
Financial Markets and Investment Strategies
Complex Systems and Time Series Analysis
Original source
May 4, 2019·Finance: Theory and Practice
3 cites
Wealth Distribution in the Bitcoin Ecosystem

A. I. Il’inskii, Z. Mierzwa

The paper deals with the problems of measuring uneven wealth distribution in the bitcoin ecosystem. All existing bitcoin distribution models depend on the analysis of bitcoin wallets and bitcoin addresses. They are based on the Bitcoin Rich List. This approach is insufficient due to the inscrutable relationships between people owning bitcoin, bitcoin wallets, and bitcoin addresses. In this paper, we used the methods of comparative analysis resulted in graphics as represented by Lorentz and Lamé curves and distribution of the Gini coefficients and the Kolkata index. We identified empirical cumulative functions of wealth distribution and the number of addresses with positive balance during the bubble and after its explosion. Approximations of the distribution of ‘poor’ and ‘rich’ addresses have been obtained and compared with the other results from the cited literature. The general public views the equality of network members as synonymous with the equal distribution of wealth among them. Emerging financial bubbles, especially in the US financial markets, lead to an increase in income inequality. However, after a bubble explodes, the inequality falls to the initial level.

Open access
Complex Systems and Time Series Analysis
Economic theories and models
Market Dynamics and Volatility
Original source
May 3, 2019·Physica A Statistical Mechanics and its Applications
22 cites
Relevant stylized facts about bitcoin: Fluctuations, first return probability, and natural phenomena

Carlo Requião da Cunha, Roberto da Silva

Bitcoin is a digital financial asset that is devoid of a central authority. This makes it distinct from traditional financial assets in a number of ways. For instance, the total number of tokens is limited and it has not explicit use value. Nonetheless, little is know whether it obeys the same stylized facts found in traditional financial assets. Here we test bitcoin for a set of these stylized facts and conclude that it behaves statistically as most of other assets. For instance, it exhibits aggregational Gaussianity and fluctuation scaling. Moreover, we show by an analogy with natural occurring quakes that bitcoin obeys both the Omori and Gutenberg-Richter laws. Finally, we show that the global persistence, originally defined for spin systems, presents a power law behavior with exponent similar to that found in stock markets.

Open access
2 source records
Complex Systems and Time Series Analysis
Financial Risk and Volatility Modeling
Market Dynamics and Volatility
Original source
May 1, 2019·AEA Papers and Proceedings
46 cites
Price Discovery in Cryptocurrency Markets

Juan Plazuelo Pascual, Carlos Tardon Rubio, Juan Toro Cebada, Angel Hernando Veciana

This document analyzes price discovery in cryptocurrency markets by comparing centralized and decentralized exchanges, as well as spot and futures markets. The study focuses first on Ethereum (ETH) and then applies a similar approach to Bitcoin (BTC). Chapter 1 outlines the theoretical framework, emphasizing the structural differences between centralized exchanges and decentralized finance mechanisms, especially Automated Market Makers (AMMs). It also explains how to construct an order book from a liquidity pool in a decentralized setting for comparison with centralized exchanges. Chapter 2 describes the methodological tools used: Hasbrouck's Information Share, Gonzalo and Granger's Permanent-Transitory decomposition, and the Hayashi-Yoshida estimator. These are applied to explore lead-lag dynamics, cointegration, and price discovery across market types. Chapter 3 presents the empirical analysis. For ETH, it compares price dynamics on Binance and Uniswap v2 over a one-year period, focusing on five key events in 2024. For BTC, it analyzes the relationship between spot and futures prices on the CME. The study estimates lead-lag effects and cointegration in both cases. Results show that centralized markets typically lead in ETH price discovery. In futures markets, while they tend to lead overall, high-volatility periods produce mixed outcomes. The findings have key implications for traders and institutions regarding liquidity, arbitrage, and market efficiency. Various metrics are used to benchmark the performance of modified AMMs and to understand the interaction between decentralized and centralized structures.

Open access
4 source records
Financial Markets and Investment Strategies
Blockchain Technology Applications and Security
Market Dynamics and Volatility
Original source
Apr 18, 2019·Journal of risk and financial management
123 cites
A Survey on Efficiency and Profitable Trading Opportunities in Cryptocurrency Markets

Νikolaos Kyriazis

This study conducts a systematic survey on whether the pricing behavior of cryptocurrencies is predictable. Thus, the Efficient Market Hypothesis is rejected and speculation is feasible via trading. We center interest on the Rescaled Range (R/S) and Detrended Fluctuation Analysis (DFA) as well as other relevant methodologies of testing long memory in returns and volatility. It is found that the majority of academic papers provides evidence for inefficiency of Bitcoin and other digital currencies of primary importance. Nevertheless, large steps towards efficiency in cryptocurrencies have been traced during the last years. This can lead to less profitable trading strategies for speculators.

Open access
2 source records
Complex Systems and Time Series Analysis
Market Dynamics and Volatility
Financial Markets and Investment Strategies
Original source
Apr 15, 2019·Economic Notes
8 cites
Bubbles and rationality in bitcoin

George Waters

Abstract Periodically collapsing rational bubbles model speculative demand in asset markets. The price and quantity of bitcoin are integrated of different orders, which is evidence of a bubble. Cointegration tests that allow for the potential presence of such bubbles with alternative proxies for fundamentals cannot reject a bubble in bitcoin.

Open access
3 source records
Complex Systems and Time Series Analysis
Market Dynamics and Volatility
Financial Markets and Investment Strategies
Original source
Apr 10, 2019·RePEc: Research Papers in Economics
0 cites
A Normative Dual-value Theory for Bitcoin and other Cryptocurrencies

Zhiyong Tu, Lan Ju

Bitcoin as well as other cryptocurrencies are all plagued by the impact from bifurcation. Since the marginal cost of bifurcation is theoretically zero, it causes the coin holders to doubt on the existence of the coin's intrinsic value. This paper suggests a normative dual-value theory to assess the fundamental value of Bitcoin. We draw on the experience from the art market, where similar replication problems are prevalent. The idea is to decompose the total value of a cryptocurrency into two parts: one is its art value and the other is its use value. The tradeoff between these two values is also analyzed, which enlightens our proposal of an image coin for Bitcoin so as to elevate its use value without sacrificing its art value. To show the general validity of the dual-value theory, we also apply it to evaluate the prospects of four major cryptocurrencies. We find this framework is helpful for both the investors and the exchanges to examine a new coin's value when it first appears in the market.

Open access
2 source records
econ.GN
Blockchain Technology Applications and Security
Complex Systems and Time Series Analysis
Original source