A347343 Positive integers k such that k with the last digit repeated is prime.

1, 19, 21, 23, 27, 31, 43, 49, 57, 59, 67, 73, 81, 87, 91, 97, 103, 127, 139, 143, 149, 151, 169, 173, 177, 181, 187, 193, 199, 201, 209, 211, 231, 233, 237, 239, 241, 247, 263, 267, 269, 271, 277, 283, 299, 301, 329, 343, 349, 351, 353, 367, 373, 383, 387

Offset: 1

Author

Paolo Xausa, Aug 27 2021

Keywords

• nonn
• base
• easy

Examples

			21 is a term because 211 is prime.
		

Links

• Michael S. Branicky, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000040, A226099.

Programs

Mathematica

upto=500;Select[Range[1,upto,2],PrimeQ[FromDigits[Join[d=IntegerDigits[#],{Last[d]}]]]&]

PARI

forprime(p=9,1e4, if(p%100%11==0, print1(p\10", "))) \\ Charles R Greathouse IV, Aug 27 2021

Python

from sympy import isprime
def ok(n): return isprime(10*n + n%10)
print(list(filter(ok, range(388)))) # Michael S. Branicky, Aug 28 2021

Formula

a(n) ~ n log n by the prime number theorem in arithmetic progressions. (These numbers are the primes mod 11, 33, 77, or 99 mod 100 with their last digit removed.) - Charles R Greathouse IV, Aug 27 2021