A032670 Digit '7' concatenated with a(n) is a prime.

1, 3, 9, 19, 27, 33, 39, 43, 51, 57, 61, 69, 73, 87, 97, 103, 109, 121, 127, 129, 151, 159, 177, 187, 193, 207, 211, 213, 219, 229, 237, 243, 247, 253, 283, 297, 307, 309, 321, 331, 333, 349, 351, 369, 393, 411, 417, 433, 451, 457, 459, 477, 481

Offset: 1

Author

Patrick De Geest, May 15 1998

Keywords

• nonn
• base

Links

• Seiichi Manyama, Table of n, a(n) for n = 1..10000

Programs

Mathematica

Select[Range[500],PrimeQ[7*10^IntegerLength[#]+#]&] (* Harvey P. Dale, Sep 16 2020 *)