A113076 Smallest prime obtained by appending one or more 7's to n, -1 if no such prime exists.

17, 277, 37, 47, 577, 67, -1, 877, 97, 107, 11777, 127, 137, -1, 157, 167, 1777, 1877, 197, 20777777, -1, 227, 2377, 2477, 257, 2677, 277, -1, 29777777777777777777777777777777777777777777777777, 307, 317, 3277777

Offset

1,1

Links

Toshitaka Suzuki, Table of n, a(n) for n = 1..94

Example

a(17) = 1777 is the smallest prime obtained by appending two 7s to 17.

Mathematica

For[n = 1, n < 50, n++, If[n/7 <= Floor[n/7], k := 1; While[Not[PrimeQ[10^k*n + (10^k - 1)*7/9]], k++ ]; Print[10^k*n + (10^k - 1)*7/9]]] (* Stefan Steinerberger, Jan 27 2006 *)
Select[Table[First[Select[Rest[FromDigits/@Table[PadRight[{x}, n, 7], {n, 50}]], PrimeQ]], {x, 35}], NumberQ]//Quiet  (* Harvey P. Dale, Jan 16 2011 *)

Crossrefs

Cf. A030432, A112386, A112394.

Sequence in context: A298373 A179093 A029811 * A159503 A375859 A245940

Adjacent sequences: A113073 A113074 A113075 * A113077 A113078 A113079

Keyword

sign,base

Author

Parthasarathy Nambi, Jan 03 2006

Extensions

More terms from Stefan Steinerberger, Jan 27 2006

Name and Data corrected by Toshitaka Suzuki, May 21 2023

Status

approved