A032684 Numbers k such that k surrounded by digit '7' is prime.

2, 5, 8, 9, 12, 17, 18, 20, 23, 24, 29, 30, 41, 45, 47, 48, 50, 51, 53, 54, 57, 60, 68, 71, 72, 75, 81, 86, 87, 90, 92, 93, 114, 116, 123, 125, 128, 131, 132, 134, 138, 143, 152, 153, 159, 164, 170, 177, 180, 183, 186, 188, 191, 194, 198, 204, 207

Offset: 1

Author

Patrick De Geest, May 15 1998

Keywords

• nonn
• base

Links

• Daniel Starodubtsev, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A032689.

Programs

Python

from sympy import isprime
print([i for i in range(200) if isprime(int('7' + str(i) + '7'))]) # Daniel Starodubtsev, Apr 05 2020