A102782 Repunit semiprimes.

111, 1111, 11111, 1111111, 11111111111, 11111111111111111, 11111111111111111111111111111111111111111111111, 11111111111111111111111111111111111111111111111111111111111

Offset

1,1

Links

Table of n, a(n) for n=1..8.

Formula

a(n) = A000042(A046413(n-1)). - Ray Chandler, Sep 06 2005

Example

a(2)=1111 because 1111=11*101, so 1111 is semiprime as well as a repunit number.

Mathematica

Select[Table[FromDigits[PadRight[{}, n, 1]], {n, 60}], PrimeOmega[#]==2&] (* Harvey P. Dale, Aug 28 2013 *)

Crossrefs

Cf. A046413 the repunit of length n has exactly 2 prime factors.

Sequence in context: A290619 A290627 A199979 * A302475 A303256 A137465

Adjacent sequences: A102779 A102780 A102781 * A102783 A102784 A102785

Keyword

base,nonn

Author

Shyam Sunder Gupta, Feb 11 2005

Status

approved