OFFSET
1,1
COMMENTS
See comments for A057472. Examined in base 12, all n must be even and all primes must be 1-primes. For example, 1249 is 881 in base 12.
The next term has 125 digits. - Harvey P. Dale, Jan 26 2019
FORMULA
EXAMPLE
a(1) = 4 since 2*5^4 - 1 = 1249 is the first prime.
MAPLE
for w to 1 do for k from 1 to 2000 do n:=2*5^k-1; if isprime(n) then printf("%d, %d", k, n) fi od od;
MATHEMATICA
Select[2*5^Range[100]-1, PrimeQ] (* Harvey P. Dale, Jan 26 2019 *)
PROG
(PARI) for(k=1, 1e3, if(ispseudoprime(p=2*5^k-1), print1(p, ", "))); \\ Altug Alkan, Sep 22 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Walter Kehowski, Jun 28 2006
STATUS
approved