From: SourceForge.net <noreply@so...>  20030814 16:50:06

Bugs item #781657, was opened at 20030801 14:30 Message generated for change (Comment added) made by macrakis You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=104933&aid=781657&group_id=4933 Category: None Group: None Status: Open Resolution: None Priority: 5 Submitted By: Barton Willis (willisb) Assigned to: Nobody/Anonymous (nobody) Summary: binomial(x,x) => 1, but binomial(1,1) => 0 Initial Comment: binomial(x,x) simplifies to 1 yet binomial(1,1) simplifies to 0. (C1) binomial(x,x); (D1) 1 (C2) binomial(1,1); (D2) 0 I agree with (d2) because: 1/(1  x) = 1 + x + x^2 + ... = sum(binomial(1,k) (x) ^k,k,0,inf) implies binomial(1,k) = (1)^k, for integers k >= 0. In the recursion relation [Knuth Vol 1, 1.2.6 Eq. (20)] binomial(r,k) = binomial(r1,k) + binomial(r1,k1) set r > 0, k > 0, and use binomial(0,0) = 1 and binomial (1,0) = 1. From this we get binomial(1,1) = 0. Also see, Knuth Vol. 1 (third edition), Section 1.2.6 Exercise 9. There may be other approaches, but I think using the recursion relations and other identities to extend the domain of the binomial function is the best method. In short, I think the simplification binomial(x,x) ==> 1 should happen only for real x with x >= 0. Barton  >Comment By: Stavros Macrakis (macrakis) Date: 20030814 12:16 Message: Logged In: YES user_id=588346 I am not sure what you mean by "there may be other approaches". binomial(a,a) should simplify to Q if and only if the double limit exists: limit binomial(x,y) = Q x>a y>a A necessary (but in general not sufficient) condition for this to exist is that the two single limits exist and are equal: limit binomial(a,y) = limit binomial(x,a) = Q y>a x>a If the limit is not welldefined, then binomial(x,x) is not well defined, and it will cause incorrect results in some case or another to arbitrarily set it to some value. I don't know if this limit is or isn't welldefined. I do know that depending on identities with unspecified domains of validity is dangerous....  You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=104933&aid=781657&group_id=4933 