A108835 Primes such that the outer 2 digits are n and n-1 and all inner digits are 7, where 0 < n < 9.

271, 877, 2777771, 8777777777, 8777777777777777, 877777777777777777777777777777777, 2777777777777777777777777777777771, 877777777777777777777777777777777777777, 877777777777777777777777777777777777777777777777777777777777777777

Offset

1,1

Comments

The left side (most significant) digit must be 2 or 8. No odd digit can work because the least significant (right side) digit would then be even and thus the number would be divisible by 2. 6 will not work because the least significant (right side) digit would be 5 and thus the number would be divisible by 5. And 4 will not work because the resulting number is always divisible by 43. Harvey P. Dale, Jun 07 2023

Links

Harvey P. Dale, Table of n, a(n) for n = 1..21

Mathematica

Select[Sort[Flatten[Table[FromDigits[Join[{l}, PadRight[{}, d, 7], {l-1}]], {l, {2, 8}}, {d, 500}]]], PrimeQ] (* Harvey P. Dale, Jun 06 2023 *)

Prog

(PARI) n10np1(n, d) = { local(x, y, k); for(x=1, n, for(k=1, 8, y=10^(x+1)*k+floor(10^x*d/9)*10+k-1; if(isprime(y), print1(y", ")) ) ) }

Crossrefs

Sequence in context: A245969 A157485 A038655 * A185668 A296038 A287232

Adjacent sequences: A108832 A108833 A108834 * A108836 A108837 A108838

Keyword

easy,nonn,base

Author

Cino Hilliard, Jul 11 2005

Extensions

More terms from Harvey P. Dale, Jul 23 2012

Status

approved