A216667 Semiprime 2-pseudoprimes of the form 10k + 7.

1387, 2047, 3277, 7957, 13747, 23377, 31417, 60787, 65077, 88357, 164737, 188057, 233017, 275887, 390937, 486737, 489997, 514447, 580337, 604117, 672487, 680627, 769567, 769757, 916327, 1092547, 1132657, 1145257, 1252697, 1293337, 1433407, 1493857, 1530787

Offset: 1

Author

Marius Coman, Sep 13 2012

Keywords

• nonn

Comments

A very interesting observation due to Peter Bala: about half of the terms from the sequence have the form p*(4*p - 3), where p is prime. For this form of Fermat pseudoprimes see the sequences A213812 and A215343.

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
• Eric Weisstein's World of Mathematics, Poulet Number

Crossrefs

Subsequence of A214305.

Cf. A001567.

Programs

PARI

list(lim)=my(v=List(),t); forprime(p=3,sqrtint(lim\=1), forprime(q=p+2,lim\p, t=p*q; if(t%10==7 && Mod(2,t)^t==2, listput(v,t)))); Set(v) \\ Charles R Greathouse IV, Jun 30 2017