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Numbers of the form k + s + 2*k*s where k is a positive integer and s is a Sundaram number (A159919).
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%I #23 Sep 08 2020 03:37:24

%S 13,22,31,37,40,49,52,58,62,67,73,76,82,85,87,94,97,103,112,115,121,

%T 122,127,130,136,137,139,142,148,157,162,166,171,172,175,178,181,184,

%U 187,192,193,199,202,211,212,214,217,220,227,229,232,237,238,241,247,253,256

%N Numbers of the form k + s + 2*k*s where k is a positive integer and s is a Sundaram number (A159919).

%C Numbers k such that bigomega(2*k + 1) >= 3. - _David A. Corneth_, Jul 15 2020

%C If a term s in A159919 is not here, 2*s+1 is a semiprime.

%e 4 is a Sundaram number, therefore 1+4+2*4*1=13 is a term, and (13*2)+1=27 is not a semiprime.

%t Select[Range[2^8], PrimeOmega[2*# + 1] >= 3 &] (* _Amiram Eldar_, Jul 15 2020 *)

%o (PARI) is(n) = bigomega(2*n + 1) >= 3 \\ _David A. Corneth_, Jul 15 2020

%Y Cf. A001222, A159919.

%K nonn,easy

%O 1,1

%A _Davide Rotondo_, Jul 15 2020

%E More terms from _David A. Corneth_, Jul 15 2020