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. Also the numbers produced by this formula are palindromic.
Table of n, a(n) for n=0..5.
For the first entry n=6, the formula produces the prime 1616161.
(PARI) /* n=number of values to test, r=repeat digits, eg., 16, 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: A221413 A145003 A097645 * A195901 A072368 A116138
Adjacent sequences: A153330 A153331 A153332 * A153334 A153335 A153336
nonn
Cino hilliard (hillcino368(AT)hotmail.com), Dec 23 2008
approved
