A109637 Numbers n such that the string 55n is prime.

7, 19, 21, 27, 31, 57, 63, 69, 73, 81, 91, 103, 109, 117, 127, 147, 163, 171, 201, 207, 213, 217, 219, 229, 243, 249, 259, 291, 313, 331, 333, 337, 339, 343, 351, 373, 381, 399, 411, 439, 441, 457, 469, 487, 501, 511, 529, 541, 547, 579, 589, 603, 609, 619

Offset

1,1

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..2000

Example

If n=7 then 55n = 557 which is a prime.
If n=81 then 55n = 5581 which is a prime.

Maple

a:= proc(n) local k; for k from 1 +`if`(n=1, 0, a(n-1))
      while not isprime(parse(""||55||k)) do od; a(n):=k
    end:
seq(a(n), n=1..100);  # Alois P. Heinz, Jul 30 2014

Mathematica

Select[Range[1, 1001, 2], PrimeQ[FromDigits[Join[{5, 5}, IntegerDigits[#]]]]&] (* Harvey P. Dale, Dec 02 2010 *)
Select[Range[700], PrimeQ[55 10^IntegerLength[#] + #] &] (* Vincenzo Librandi, Jul 30 2014 *)

Prog

(Magma) [n: n in [1..600] | IsPrime(Seqint(Intseq(n) cat [5, 5])) ]; // Vincenzo Librandi, Jul 30 2014

Crossrefs

Sequence in context: A102167 A214525 A372539 * A039513 A125265 A023152

Adjacent sequences: A109634 A109635 A109636 * A109638 A109639 A109640

Keyword

nonn,base

Author

Parthasarathy Nambi, Aug 30 2005

Extensions

More terms from Vincenzo Librandi, Mar 27 2010

Status

approved