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 May 26 answered Rijndael S-Box Permutation Group May 26 comment “Casual” mathematical facts with practical consequences @Jim: Yup, that's why I used the word "seem". Anyway, I think it's pretty clever. May 25 comment “Casual” mathematical facts with practical consequences The puzzle is to explain why these two differently-sized rectangles seem to be both partitioned in to the same 4 polygons. Knuth claims this was a favorite puzzle of Lewis Carroll (See "Concrete Mathematics", Graham, Knuth, Patashnik, pg 293) May 25 answered “Casual” mathematical facts with practical consequences May 25 revised “Casual” mathematical facts with practical consequences added 3 characters in body May 24 comment Is the factorization problem harder than RSA factorization ($n = pq$)? The answer is trivially "yes" since you didn't limit computational power in any way, and factorization is certainly decidable. May 23 awarded Nice Question May 23 revised How valid is the concern over narrow pipe cryptographic hash function designs? added 328 characters in body May 23 comment How valid is the concern over narrow pipe cryptographic hash function designs? @Pete: Point well-taken. However, I prefer remaining anonymous on these forums, so I'll have to live with the loss-of-value engendered as a result. May 23 answered How valid is the concern over narrow pipe cryptographic hash function designs? May 23 comment Partial sum over $M$, of ${m+j \choose M} {1-M \choose m+i-M}$? Although I don't know if your summation is handled by the methods in the following book, you might want to check it out. The methods here are often called "revolutionary" as regards summations of binomial coefficients: math.upenn.edu/~wilf/AeqB.html May 21 comment Subset sum problem is NP-complete? Yes, thanks. Corrected. May 21 revised Subset sum problem is NP-complete? added 2 characters in body May 21 answered Subset sum problem is NP-complete? May 21 revised What are a , b and c? edited tags May 21 answered Can a number have a prime factor that isn't a part of the number's prime factorization? May 20 comment Another pigeonhole principle question @Arvin: Yes, c(7,3) is correct. I was leaving that part for you to work out, but I see other respondents did the whole problem shortly after. Ah well. May 20 awarded Good Question May 20 answered Another pigeonhole principle question May 20 revised Preparation for Putnam? edited tags