OFFSET

0,3

COMMENTS

a(3) has 111 digits.

As in the case of A077585, where a necessary condition for a term to be prime is that its index is a Mersenne prime, a necessary (but not sufficient) condition for a term of this sequence to be prime is that the number of ones is a repunit prime, i.e., A055642(a(n)) must be a term of A004022.

MATHEMATICA

Table[Nest[FromDigits@ ConstantArray[1, #] &, n, 2], {n, 0, 3}] (* Michael De Vlieger, Jul 30 2017 *)

PROG

(PARI) a002275(n) = (10^n-1)/9

a(n) = a002275(a002275(n))

CROSSREFS

KEYWORD

nonn,base,easy,bref

AUTHOR

Felix FrÃ¶hlich, Jul 29 2017

STATUS

approved