cp's OEIS Frontend

A200677 Smallest semiprime such that the sum of the two prime factors equals n, or zero if impossible.

%I A200677 #29 Nov 22 2021 10:03:28
%S A200677 0,0,0,4,6,9,10,15,14,21,0,35,22,33,26,39,0,65,34,51,38,57,0,95,46,69,
%T A200677 0,115,0,161,58,87,62,93,0,155,0,217,74,111,0,185,82,123,86,129,0,215,
%U A200677 94,141,0,235,0,329,106,159,0,265,0,371,118,177,122,183,0
%N A200677 Smallest semiprime such that the sum of the two prime factors equals n, or zero if impossible.
%C A200677 For n > 3, a(n) = 0 if n-2 is an odd composite.
%C A200677 The sequence without zeros is a subsequence of A189553. - _Manfred Scheucher_, Aug 08 2015
%C A200677 The two prime factors are not necessarily distinct; a(6) = 9, both of whose prime factors are 3s. - _Jon E. Schoenfield_, Aug 09 2015
%F A200677 a(A014091(n)) > 0; a(A014092(n)) = 0. - _Michel Marcus_, Aug 10 2015
%e A200677 a(10) = 21 because 21 = 3*7 and 3+7 = 10, and there is no semiprime smaller than 21 whose two prime factors sum to 10.
%p A200677 with(numtheory):for n from 1 to 65 do:ii:=0:for k from 1 to 1000 while(ii=0)do:m1:=bigomega(k):x:=factorset(k): m2:=nops(x):if m1=2 and m2=2 and x[1]+x[2]= n or m1=2 and m2=1 and 2*x[1]= n then ii:=1: printf(`%d, `,k):else fi:od:if ii=0 then printf(`%d, `,0):else fi:od:
%Y A200677 Cf. A001358, A014091, A014092, A135093.
%K A200677 nonn
%O A200677 1,4
%A A200677 _Michel Lagneau_, Nov 20 2011
%E A200677 Edited by _Jon E. Schoenfield_ and _Manfred Scheucher_, Aug 09 2015