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.
%I A260648 #22 May 03 2017 07:54:39 %S A260648 1,2,0,1,2,1,1,2,0,1,1,2,1,0,1,1,1,1,2,0,1,2,2,0,1,2,0,1,1,1,1,0,1,1, %T A260648 2,1,0,2,1,2,1,0,1,1,0,2,2,1,1,1,0,1,1,1,2,1,1,0,1,1,2,1,1,1,0,2,1,1, %U A260648 1,2,0,0,2,0,1,1,2,1,0,1,2,1,1,1,1,1,1,2,0,1,1,1,1,1,0,0,1,3 %N A260648 Number of distinct prime divisors p of the n-th composite number c such that gpf(c - p) = p, where gpf = greatest prime factor (A006530). %C A260648 a(n) gives the number of times that the n-th composite number occurs in A070229. %H A260648 Robert Israel, <a href="/A260648/b260648.txt">Table of n, a(n) for n = 1..10000</a> %e A260648 a(8) = 2 since the distinct prime divisors of A002808(8) = 15 are 3 and 5, A006530(15 - 3) = 3 and A006530(15 - 5) = 5, so all prime 3 and 5 are to be considered. %p A260648 N:= 1000: # to consider composites <= N %p A260648 f:= proc(c) local p, t; %p A260648 if isprime(c) then return NULL fi; %p A260648 nops(select(p -> max(numtheory:-factorset(c/p-1))<=p, numtheory:-factorset(c))) %p A260648 end proc: %p A260648 map(f, [$4..N]); # _Robert Israel_, May 02 2017 %Y A260648 Cf. A002808 (composite), A006530 (gpf). %K A260648 nonn %O A260648 1,2 %A A260648 _Gionata Neri_, Nov 12 2015 %E A260648 a(87) corrected by _Robert Israel_, May 02 2017