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A099481 Numbers k such that 2^k - k^2 is a semiprime. 2
11, 13, 15, 21, 23, 37, 39, 41, 43, 47, 49, 55, 67, 75, 103, 105, 133, 147, 153, 161, 163, 177, 201, 209, 221, 239, 249, 263, 311, 335, 355, 397, 413, 421, 437, 447, 583, 617, 775, 807 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
The smaller prime factor of the 125-digit semiprime 2^413 - 413^2 has 40 digits; for the 127-digit semiprime 2^421 - 421^2 the smaller prime factor has 45 digits. The next term is >= 583. - Hugo Pfoertner, Oct 14 2007
The factorization of the 176-decimal-digit composite 2^583 - 583^2 using SNFS in YAFU took 55000 seconds on 4 cores of an i5-2400 CPU @ 3.10GHz. a(38) >= 617. - Hugo Pfoertner, Jul 23 2019
a(41) >= 827. - Hugo Pfoertner, Jul 26 2019
LINKS
EXAMPLE
a(1) = 11 because 2^11 - 11^2 = 1927 = 41*47.
CROSSREFS
Cf. A024012 (2^n-n^2), A099482 (semiprimes of the form 2^n-n^2), A072180 (2^n-n^2 is prime), A075896 (primes of the form 2^n-n^2).
Sequence in context: A260826 A289699 A049722 * A254412 A215778 A211021
KEYWORD
nonn,more,hard
AUTHOR
Hugo Pfoertner, Oct 18 2004
EXTENSIONS
More terms from Hugo Pfoertner, Oct 14 2007
a(37)-a(40) from Hugo Pfoertner, Jul 26 2019
STATUS
approved

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Last modified April 23 07:11 EDT 2024. Contains 371905 sequences. (Running on oeis4.)