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A099441 Numbers n such that A000295(n) = 2^n-n-1 is a semiprime. 3
3, 5, 6, 8, 9, 12, 30, 32, 33, 46, 52, 62, 74, 76, 86, 98, 130, 134, 154, 228, 230, 242, 256, 266, 346, 352, 382, 412, 428, 474, 488 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A candidate for the next term is a(32) = 634. 2^634-635 is composite with 191 decimal digits and unknown factorization. - Hugo Pfoertner, Aug 13 2007

LINKS

Table of n, a(n) for n=1..31.

Dario Alpern, Factorization using the Elliptic Curve Method.

Status of 2^634-635 in factordb.com.

EXAMPLE

a(3) = 6 because 2^6-6-1 = 57 = 3*19 is a semiprime.

CROSSREFS

Cf. A000295 2^n-n-1 (column 2 of the Eulerian numbers), A099439 2^n-n-1 is prime, A099440 primes in A000295, A099442 semiprimes in A000295.

Sequence in context: A295581 A164902 A278996 * A129359 A110801 A078339

Adjacent sequences:  A099438 A099439 A099440 * A099442 A099443 A099444

KEYWORD

hard,more,nonn

AUTHOR

Hugo Pfoertner, Oct 18 2004

EXTENSIONS

More terms from Hugo Pfoertner, Aug 13 2007

a(24) corrected by Hugo Pfoertner, Sep 07 2017

STATUS

approved

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Last modified July 29 05:20 EDT 2021. Contains 346340 sequences. (Running on oeis4.)