A320585 Primes whose first digit is prime.

2, 3, 5, 7, 23, 29, 31, 37, 53, 59, 71, 73, 79, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 503, 509, 521, 523, 541, 547, 557, 563, 569, 571

Offset: 1

Author

Kritsada Moomuang, Oct 16 2018

Keywords

• nonn
• base

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Intersection of A000040 and A320584 (numbers whose first digit is prime).

Programs

Maple

2,3,5,7,seq(seq(op(select(isprime,[seq(i*10^(d-1)+j,j=1..10^(d-1)-1,2)])),i=[2,3,5,7]),d=1..4); # Robert Israel, Apr 02 2020

Mathematica

Select[Range[575], PrimeQ[#] && PrimeQ[IntegerDigits[#][[1]]] &] (* Amiram Eldar, Nov 11 2018 *)

PARI

isok(n) = isprime(n) && isprime(digits(n)[1]); \\ Michel Marcus, Oct 17 2018