A356424 - OEIS

%I #17 Jan 16 2023 04:28:35

%S 9,46,111,559,1639,3961,4699,7291,11629,12871,23329,30691,32689,41311,

%T 48439,85879,114211,129889,142309,159751,262081,267859,310069,342109,

%U 389611,418141,486019,542341,584461,619291,729829,758881,923401,967051,1011709,1104049,1163809

%N 9-gonal numbers that are semiprimes.

%H Robert Israel, <a href="/A356424/b356424.txt">Table of n, a(n) for n = 1..10000</a>

%e        9 =   3 *    3 =   2*(7*  2 - 5)/2.

%e     3961 =  17 *  233 =  34*(7* 34 - 5)/2.

%e    41311 = 109 *  379 = 109*(7*109 - 5)/2.

%e   758881 = 233 * 3257 = 466*(7*466 - 5)/2.

%p R:= NULL: count:= 0:

%p for i from 1 while count < 100 do

%p   v:= i*(7*i-5)/2;

%p if numtheory:-bigomega(v) = 2 then R:= R,v; count:= count+1 fi

%p od:

%p R; # _Robert Israel_, Jan 15 2023

%t Select[Table[n*(7*n - 5)/2, {n, 1, 600}], PrimeOmega[#] == 2 &] (* _Amiram Eldar_, Aug 07 2022 *)

%Y Intersection of A001106 and A001358.

%K nonn

%O 1,1

%A _Massimo Kofler_, Aug 07 2022