ev, subst, or at?



Hi there:

What is the difference between the commands ev, subst, and at?  If I need to substitute a numerical value for a free variable in a simple formula, can they be used interchangeably?  This appears to be the case in the examples below, but I want to make sure I am not missing something subtle.

(%i14) ev(x^2 + 7*y, x=6);
(%o14) 7y+36

(%i15) subst(x=6, x^2 + 7*y);
(%o15) 7y+36

(%i16) at(x^2 + 7*y, x=6);
(%o16) 7y+36

Thanks for your help!

Jorge

--
Dr. Jorge Alberto Calvo
Associate Professor of Mathematics
Department of Mathematics and Physics
Ave Maria University

Phone: (239) 280-1608
Email: jorge.calvo at avemaria.edu
Web: http://sites.google.com/site/jorgealbertocalvo