A123608 - OEIS

A123608

Numbers k such that k, k+1 and 2*k+1 are composite.

24, 25, 27, 32, 34, 38, 45, 49, 55, 57, 62, 64, 76, 77, 80, 84, 85, 87, 91, 92, 93, 94, 104, 110, 115, 117, 118, 121, 122, 123, 124, 129, 132, 133, 142, 143, 144, 145, 147, 152, 154, 159, 160, 161, 164, 169, 170, 171, 175, 177, 182, 184, 185, 187, 188, 195, 201

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OFFSET

1,1

COMMENTS

Also numbers n such that A098735(n) = 1. Also numbers n such that A098077(n) divides n!. Note that n, n+1 and 2n+1 are all primes only for n = 2.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

MATHEMATICA

Select[Range[500], !PrimeQ[ # ]&&!PrimeQ[ #+1]&&!PrimeQ[2#+1]&]

CROSSREFS

Cf. A098735, A098077.

Sequence in context: A271232 A167458 A066024 * A141587 A296866 A241890

Adjacent sequences: A123605 A123606 A123607 * A123609 A123610 A123611

KEYWORD

nonn

AUTHOR

Alexander Adamchuk, Nov 15 2006

STATUS

approved