A123608 - OEIS

%I #7 Mar 29 2020 02:19:25

%S 24,25,27,32,34,38,45,49,55,57,62,64,76,77,80,84,85,87,91,92,93,94,

%T 104,110,115,117,118,121,122,123,124,129,132,133,142,143,144,145,147,

%U 152,154,159,160,161,164,169,170,171,175,177,182,184,185,187,188,195,201

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

%C 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.

%H Amiram Eldar, <a href="/A123608/b123608.txt">Table of n, a(n) for n = 1..10000</a>

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

%Y Cf. A098735, A098077.

%K nonn

%O 1,1

%A _Alexander Adamchuk_, Nov 15 2006