A320584 Numbers whose first digit is prime.

2, 3, 5, 7, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217

Offset: 1

Author

Kritsada Moomuang, Oct 16 2018

Keywords

• nonn
• base

Crossrefs

Supersequence of A320585 (primes whose first digit is prime).

Cf. A000030, A000040.

Programs

Mathematica

Select[Range@ 218, PrimeQ@ First@ IntegerDigits@ # &] (* Michael De Vlieger, Nov 05 2018 *)

PARI

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

PARI

a(n) = n--; my (w=1); while (n >= 4*w, n -= 4*w; w*=10); w*prime(1+n\w) + (n%w) \\ Rémy Sigrist, Nov 16 2018