|
%I #16 Jun 07 2023 11:55:59
%S 271,877,2777771,8777777777,8777777777777777,
%T 877777777777777777777777777777777,2777777777777777777777777777777771,
%U 877777777777777777777777777777777777777,877777777777777777777777777777777777777777777777777777777777777777
%N Primes such that the outer 2 digits are n and n-1 and all inner digits are 7, where 0 < n < 9.
%C 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
%H Harvey P. Dale, <a href="/A108835/b108835.txt">Table of n, a(n) for n = 1..21</a>
%t Select[Sort[Flatten[Table[FromDigits[Join[{l},PadRight[{},d,7],{l-1}]],{l,{2,8}},{d,500}]]],PrimeQ] (* _Harvey P. Dale_, Jun 06 2023 *)
%o (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",")) ) ) }
%K easy,nonn,base
%O 1,1
%A _Cino Hilliard_, Jul 11 2005
%E More terms from _Harvey P. Dale_, Jul 23 2012
|
