0,1

These numbers are always even. If n is odd, then 10^n-1 produces a number with an odd number of 9's which 99 does not divide. a(10) = 14141414141 which is prime. Also these numbers are palindromic.

Table of n, a(n) for n=0..4.

(PARI) /* n=number of values to test, r=repeat digits, eg., 14, 121, 177, 1234, etc. d = last digit appended to the end */

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", ")))

Sequence in context: A055055 A222998 A221526 * A290202 A203149 A117654

Adjacent sequences: A153328 A153329 A153330 * A153332 A153333 A153334

nonn,base

Cino Hilliard, Dec 23 2008

approved