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A290390 Double repunit numbers: repunits with repunit indices. 0
0, 1, 11111111111, 111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111 (list; graph; refs; listen; history; text; internal format)
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.

Are there any primes in this sequence? In other words, is there a term of A004022 that is also a term of A004023?

Second sequence in the hierarchy of sequences obtained by successive numbers of nestings of the form A002275(...A002275(n)...). All higher order sequences in this hierarchy grow much too fast to be included in the OEIS.

LINKS

Table of n, a(n) for n=0..3.

FORMULA

a(n) = A002275(A002275(n)).

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

Cf. A002275, A004022, A004023, A077585.

Sequence in context: A295464 A095426 A038453 * A183709 A247312 A204780

Adjacent sequences:  A290387 A290388 A290389 * A290391 A290392 A290393

KEYWORD

nonn,base,easy,bref

AUTHOR

Felix Fröhlich, Jul 29 2017

STATUS

approved

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Last modified October 20 09:40 EDT 2021. Contains 348100 sequences. (Running on oeis4.)