A381510 Smaller of two consecutive primes p and q, both ending with 7, such that q - p = 10n, or -1 if no such primes exist.

337, 887, 4297, 33247, 31907, 124367, 218287, 1122287, 1964987, 1313467, 1468277, 7160227, 5518687, 16525757, 13626257, 71880637, 27915737, 17051707, 394059907, 566348087, 252314747, 472865287, 1289694257, 633418787, 1588640437, 944192807, 1391048047, 7059848287

Offset: 1

Author

Jean-Marc Rebert, Feb 25 2025

Keywords

• nonn
• base

Examples

			a(1) = 337, because 337 and 337 + 10 = 347 are two consecutive primes with the same last digit 7 and no smaller prime has this property.
		

Crossrefs

Cf. A101232, A140791, A380785, A381372.

Programs

PARI

a(n) = my(p=7); while (!isprime(p) || ((nextprime(p+1)-p) != 10*n), p+=10); p; \\ Michel Marcus, Feb 25 2025

Python

from sympy import nextprime, isprime
def A381510(n):
    p = 17
    while (q:=nextprime(p)):
        if q-p == 10*n:
            return p
        p = q+9-(q+2)%10
        while not isprime(p):
            p += 10 # Chai Wah Wu, Mar 09 2025