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A123608
Numbers k such that k, k+1 and 2*k+1 are composite.
2
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
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
MATHEMATICA
Select[Range[500], !PrimeQ[ # ]&&!PrimeQ[ #+1]&&!PrimeQ[2#+1]&]
CROSSREFS
Sequence in context: A271232 A167458 A066024 * A141587 A296866 A241890
KEYWORD
nonn
AUTHOR
Alexander Adamchuk, Nov 15 2006
STATUS
approved