cp's OEIS Frontend

A131368 Coprime semiprimes: a(n-1) and a(n) are the closest coprime semiprimes.

%I A131368 #13 Jun 11 2023 14:16:31
%S A131368 4,9,10,21,22,25,26,33,34,35,38,39,46,49,51,55,57,58,65,69,74,77,82,
%T A131368 85,86,87,91,93,94,95,106,111,115,118,119,121,122,123,133,134,141,142,
%U A131368 143,145,146,155,158,159,161,166,169,177,178,183,185,187,194,201,202,203
%N A131368 Coprime semiprimes: a(n-1) and a(n) are the closest coprime semiprimes.
%C A131368 The slowest increasing sequence of semiprimes with coprime neighbor terms.
%H A131368 Robert Israel, <a href="/A131368/b131368.txt">Table of n, a(n) for n = 1..10000</a>
%F A131368 gcd(a(n-1), a(n)) = 1.
%p A131368 A[1]:= 4: t:= 4: count:= 1:
%p A131368 for x from 5 while count < 100 do
%p A131368   if igcd(x, t) = 1 then
%p A131368     if numtheory:-bigomega(x)=2 then
%p A131368       count:= count+1;
%p A131368       A[count]:= x;
%p A131368       t:= x;
%p A131368     fi
%p A131368   fi
%p A131368 od:
%p A131368 seq(A[i], i=1..100); # _Robert Israel_, Jun 11 2023
%o A131368 (PARI) lista(nn) = {my(vsp = select((x->(bigomega(x) == 2)), vector(nn, k, k)), i = 1, last); while (i <= #vsp, print1(vsp[i], ", "); last = vsp[i]; while(gcd(vsp[i], last) != 1, i++; if (i>#vsp, break)););} \\ _Michel Marcus_, Jun 25 2019
%Y A131368 Cf. A051884, A061766, A107621, A109412.
%Y A131368 Subsequence of A001358 (semiprimes).
%K A131368 nonn
%O A131368 1,1
%A A131368 _Zak Seidov_, Sep 30 2007