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A242332 Numbers k such that k^2 + 4 is a semiprime. 4
0, 9, 19, 21, 23, 25, 31, 41, 43, 51, 53, 55, 63, 69, 71, 75, 77, 79, 83, 91, 93, 105, 107, 109, 113, 119, 123, 129, 131, 133, 143, 145, 149, 151, 153, 157, 165, 171, 173, 175, 181, 185, 187, 191, 195, 197, 201, 209, 221, 223, 225, 227, 241, 249, 251, 257, 259 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The semiprimes of this form are: 4, 85, 365, 445, 533, 629, 965, 1685, 1853, 2605, 2813, 3029, 3973, 4765, 5045, 5629, 5933, 6245, ...

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

MATHEMATICA

Select[Range[0, 300], PrimeOmega[#^2 + 4] == 2 &]

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [0..300] | IsSemiprime(s) where s is n^2+4];

CROSSREFS

Cf. A005473, A007591, A085722, A242330, A242331, A242333.

Sequence in context: A288392 A157140 A244538 * A075981 A079368 A167529

Adjacent sequences:  A242329 A242330 A242331 * A242333 A242334 A242335

KEYWORD

nonn

AUTHOR

Vincenzo Librandi, May 14 2014

STATUS

approved

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Last modified July 4 15:25 EDT 2020. Contains 335448 sequences. (Running on oeis4.)