cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A093451 Number of distinct prime divisors of Product_{k=1+(n-1)n/2..n(n+1)/2} k (i.e., of 1, 2*3, 4*5*6, 7*8*9*10, ...).

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%I A093451 #18 Jan 09 2024 16:19:10
%S A093451 0,2,3,4,6,6,7,8,10,10,11,13,13,14,16,15,18,17,20,19,22,21,22,24,24,
%T A093451 26,26,27,30,28,30,31,32,33,33,36,35,36,38,39,39,39,43,41,43,44,44,47,
%U A093451 45,49,48,48,52,49,53,53,54,54,55,58,55,60,59,59,62,60,65,64,64,65,66,68
%N A093451 Number of distinct prime divisors of Product_{k=1+(n-1)n/2..n(n+1)/2} k (i.e., of 1, 2*3, 4*5*6, 7*8*9*10, ...).
%H A093451 David A. Corneth, <a href="/A093451/b093451.txt">Table of n, a(n) for n = 1..10000</a>
%F A093451 a(n) = A001221(A057003(n)). - _Michel Marcus_, Jul 29 2017
%e A093451 a(7) = 7 as the prime divisors of the product 22*23*24*25*26*27*28 are 2,3,5,7,11,13 and 23.
%p A093451 with(numtheory): a:=n->nops(factorset(product(k,k=1+n*(n-1)/2..n*(n+1)/2))): seq(a(n),n=1..80); # _Emeric Deutsch_, Feb 05 2006
%t A093451 With[{nn=75},PrimeNu[#]&/@Times@@@TakeList[Range[(nn(nn+1))/2],Range[ nn]]] (* _Harvey P. Dale_, Sep 01 2021 *)
%o A093451 (PARI) a(n) = { my(b=binomial(n,2)+1, bp1=binomial(n+1,2), res = primepi(n)); forprime(p = n + 1, bp1, bp = b%p; if(bp > bp1 % p || bp == 0, res++ ) ); res } \\ _David A. Corneth_, Sep 01 2021
%Y A093451 Cf. A001221, A057003.
%K A093451 nonn
%O A093451 1,2
%A A093451 _Amarnath Murthy_, Apr 03 2004
%E A093451 Corrected and extended by _Emeric Deutsch_, Feb 05 2006