Baily–Borel compactification
Example
If C is the quotient of the upper half plane by a congruence subgroup of SL2 (Z ), then the Baily–Borel compactification of C is formed by adding a finite number of cusps to it.
References
Baily, Walter L. Jr.; Borel, Armand (1964), "On the compactification of arithmetically defined quotients of bounded symmetric domains" , Bulletin of the American Mathematical Society , 70 (4): 588– 593, doi :10.1090/S0002-9904-1964-11207-6 , MR 0168802
Baily, W.L.; Borel, A. (1966), "Compactification of arithmetic quotients of bounded symmetric domains", Annals of Mathematics , 2, 84 (3), Annals of Mathematics: 442– 528, doi :10.2307/1970457 , JSTOR 1970457 , MR 0216035
Gordon, B. Brent (2001) [1994], "Baily–Borel compactification" , Encyclopedia of Mathematics , EMS Press