A320256 k-digit primes with the same even digit repeated k-1 times and a single odd digit.

3, 5, 7, 23, 29, 41, 43, 47, 61, 67, 83, 89, 223, 227, 229, 443, 449, 661, 881, 883, 887, 2221, 4441, 4447, 6661, 8887, 22229, 44449, 88883, 444443, 444449, 666667, 888887, 22222223, 66666667, 88888883, 222222227, 444444443, 666666667, 888888883, 888888887

Offset

1,1

Comments

For the resulting number to be prime, the rightmost digit must be the odd one. - Michel Marcus, Oct 11 2018

Links

Alois P. Heinz, Table of n, a(n) for n = 1..132

Example

3, 5, 7 are in the sequence for k = 1.
229 is in the sequence because it is a 3-digit prime with the first 3-1 digits repeating even (2) and the last digit odd (9). - David A. Corneth, Oct 10 2018

Mathematica

Join[{3, 5, 7}, Select[Flatten@ Table[{1, 3, 7, 9} + 10 FromDigits@ ConstantArray[k, n], {n, 9}, {k, Range[2, 8, 2]}], PrimeQ]] (* Michael De Vlieger, Oct 31 2018 *)

Prog

(PARI) first(n) = {n = max(n, 3); my(t = 3, res = List([3, 5, 7])); print1("3, 5, 7, "); for(i=1, oo, k=(10^i - 1) / 9; forstep(f = 2, 8, 2, forstep(d=1, 9, 2, c = 10 * f * k + d; if(isprime(c), print1(c", "); listput(res, c); t++; if(t>=n, return(res))))))} \\ David A. Corneth, Oct 10 2018

Crossrefs

Cf. A055558, A068690, A105975, A141311, A154764.

Sequence in context: A296922 A119450 A154764 * A101773 A057182 A038916

Adjacent sequences: A320253 A320254 A320255 * A320257 A320258 A320259

Keyword

nonn,base

Author

Enrique Navarrete, Oct 08 2018

Extensions

More terms from Michel Marcus, Oct 10 2018

Status

approved