login
Numbers ending in 1 which are not prime.
1

%I #19 Jan 21 2021 10:46:27

%S 1,21,51,81,91,111,121,141,161,171,201,221,231,261,291,301,321,341,

%T 351,361,371,381,391,411,441,451,471,481,501,511,531,551,561,581,591,

%U 611,621,651,671,681,711,721,731,741,771,781,791,801,831,841,851,861,871

%N Numbers ending in 1 which are not prime.

%C Nonprime numbers of the form k*10+1. - _Juri-Stepan Gerasimov_, Oct 14 2009

%e a(4) = 91 may look prime to some, but is composite.

%p remove(isprime, [10*j+1$j=0..99])[]; # _Alois P. Heinz_, Jan 21 2021

%o (PARI) isok(n) = ((n % 10) == 1) && (! isprime(n)) \\ _Michel Marcus_, Jul 26 2013

%o (Python)

%o from sympy import isprime

%o def aupto(lim): return [m for m in range(1, lim+1, 10) if not isprime(m)]

%o print(aupto(871)) # _Michael S. Branicky_, Jan 21 2021

%Y Cf. A002808, A017281, A153275.

%K easy,nonn,base

%O 1,2

%A _Enoch Haga_, Feb 29 2000