OFFSET
0,2
COMMENTS
Least inverse of A034093. - Charles R Greathouse IV, May 01 2012
a(10) = 642 and a(11) = 1682. - Charles R Greathouse IV, May 03 2012
a(>11) > 5000. - Robert Price, Nov 06 2023
LINKS
Chris Caldwell, Below are all of the 12-digit Near-Repunit primes:
Chris Caldwell, Repunits
EXAMPLE
a(5) = 12 because R_12 = (10^12 -1)/9 = 111111111111 and from this number, by changing just one digit from 1 to 0, out of the eleven candidates, 111111111101, 111111110111, 111111011111, 111011111111 and 101111111111 are primes.
MATHEMATICA
a = Table[0, {10} ]; Do[p = IntegerDigits[ (10^n - 1)/9]; c = 0; Do[ If[ q = FromDigits[ ReplacePart[p, 0, i]]; PrimeQ[q], c++ ], {i, 2, n} ]; If[ a[[c + 1]] == 0, a[[c + 1]] = n], {n, 1, 400} ]; a
PROG
(PARI) a(n)=my(k=1); while(sum(i=1, k-2, ispseudoprime(10^k\9-10^i)) != n, k++); k \\ Charles R Greathouse IV, May 01 2012
CROSSREFS
KEYWORD
more,nonn,base
AUTHOR
Robert G. Wilson v, Nov 19 2001
EXTENSIONS
a(6) from Charles R Greathouse IV, May 01 2012
a(9) from Robert Price, Nov 04 2023
a(10)-a(11) from comments and verified by Robert Price, Nov 04 2023
STATUS
approved