A283209 Primes of the form 299...977...7 with at least one 9 and one 7.

299777, 299977, 29999777, 299999977, 2999999777, 299999999777, 2999977777777, 299999999999977, 2999999999977777777, 2999999999999999977, 299999999999977777777, 299999999999999999977, 2999999999999999777777

Offset

1,1

Comments

If the number of 7's modulo 3 equals 1, the corresponding 29..97..7 term cannot be in sequence because it is divisible by 3.

If the number of 9's modulo 6 equals 5, the corresponding 29..97..7 term cannot be in sequence because 299999 and 999999 are divisible by 7.

If the number of 7's and the number of 9's are both odd, the corresponding 29..97..7 term cannot be in sequence because it is divisible by 11.

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..868

Mathematica

Sort@ Select[Map[FromDigits@ Join[{2}, ConstantArray[9, #1], ConstantArray[7, #2]] & @@ # &, Select[Tuples[Range@ 20, 2], Times @@ Boole@ Map[OddQ, #] == 0 &]], PrimeQ] (* Michael De Vlieger, Mar 06 2017 *)

Prog

(PARI) do(n)=my(v=List(), p=29, q); for(d=3, n, p=10*p+7; q=p; forstep(i=d-3, 1, -1, if(ispseudoprime(q+=2*10^i), listput(v, q)))); Vec(v) \\ Charles R Greathouse IV, Mar 06 2017

Crossrefs

Sequence in context: A022246 A227700 A050257 * A113939 A023686 A250506

Adjacent sequences: A283206 A283207 A283208 * A283210 A283211 A283212

Keyword

nonn,base

Author

FUNG Cheok Yin, Mar 03 2017

Extensions

More terms from Charles R Greathouse IV, Mar 06 2017

Status

approved