A122488 - OEIS

A122488

Numbers k such that 1 + 2k + 3k^2 is semiprime.

1, 3, 4, 5, 6, 8, 9, 10, 11, 15, 20, 21, 22, 29, 30, 33, 35, 36, 40, 42, 45, 47, 48, 50, 54, 57, 59, 60, 64, 66, 69, 72, 75, 76, 77, 78, 80, 81, 82, 84, 86, 89, 95, 96, 100, 102, 108, 110, 111, 112, 113, 116, 123, 125, 130, 131, 132, 134, 136, 143, 146, 147, 148, 152, 153

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OFFSET

1,2

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

k=3 is a term because 1 + 2k + 3k^2 = 34 = 2*17 (semiprime);

k=4 is a term because 1 + 2k + 3k^2 = 57 = 3*19 (semiprime); etc.

MAPLE

filter:= n -> numtheory:-bigomega(1+2*n+3*n^2)=2:

select(filter, [$1..200]); # Robert Israel, Nov 03 2022

MATHEMATICA

Select[Range[160], PrimeOmega[1+2#+3#^2]==2&] (* Harvey P. Dale, Aug 22 2011 *)

CROSSREFS

Cf. A086285 (numbers k such that 1 + 2k + 3k^2 is prime).

Sequence in context: A266728 A039132 A187970 * A241403 A287727 A298606

Adjacent sequences: A122485 A122486 A122487 * A122489 A122490 A122491

KEYWORD

nonn

AUTHOR

Zak Seidov, Oct 20 2006

STATUS

approved