A153333 - OEIS

%I #15 Dec 12 2021 22:51:57

%S 6,54,108,144,228,960

%N Numbers k such that (10^k - 1)*160/99 + 1 is prime.

%C These numbers are always even. If k is odd, then 10^k - 1 produces a number with an odd number of 9's which 99 does not divide. Also the numbers produced by this formula are palindromic.

%e For the first entry, k=6, the formula produces the prime 1616161.

%o (PARI) /* n=number of values to test, r=repeat digits, e.g., 16, 121, 177, 1234, etc. d = last digit appended to the end */

%o repr(n,r,d) = ln=length(Str(r));for(x=0,n,y=(10^(ln*x)-1)*10*r/(10^ln-1)+1;if(ispseudoprime(y),print1(ln*x",")))

%K nonn,base,more

%O 1,1

%A _Cino Hilliard_, Dec 23 2008