Uses the getYahooData function of the TTR package
to retrieve the adjusted closing prices of the two securities over
the specified date range. Then, constructs an Engle-Granger cointegration
model from this data, and returns it.
References
Engle, R. F. and C. W. Granger. (1987)
Co-integration and error correction: representation, estimation, and testing.
Econometrica, 251-276.