A053178 Numbers ending in 1 which are not prime.

1, 21, 51, 81, 91, 111, 121, 141, 161, 171, 201, 221, 231, 261, 291, 301, 321, 341, 351, 361, 371, 381, 391, 411, 441, 451, 471, 481, 501, 511, 531, 551, 561, 581, 591, 611, 621, 651, 671, 681, 711, 721, 731, 741, 771, 781, 791, 801, 831, 841, 851, 861, 871

Offset: 1

Author

Enoch Haga, Feb 29 2000

Keywords

• easy
• nonn
• base

Comments

Nonprime numbers of the form k*10+1. - Juri-Stepan Gerasimov, Oct 14 2009

Examples

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

Crossrefs

Cf. A002808, A017281, A153275.

Programs

Maple

remove(isprime, [10*j+1$j=0..99])[];  # Alois P. Heinz, Jan 21 2021

PARI

isok(n) = ((n % 10) == 1) && (! isprime(n)) \\ Michel Marcus, Jul 26 2013

Python

from sympy import isprime
def aupto(lim): return [m for m in range(1, lim+1, 10) if not isprime(m)]
print(aupto(871)) # Michael S. Branicky, Jan 21 2021