

A083340


Numbers n such that A020549(n)=(n!)^2+1 is a semiprime.


2



6, 7, 8, 12, 15, 16, 17, 18, 19, 28, 29, 41, 45, 53, 55, 61, 73
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OFFSET

1,1


COMMENTS

The smaller of the two prime factors is given in A083341. The next candidates for a continuation are 55 and 61. (55!)^2 + 1 is composite with 147 decimal digits and unknown factorization.
(55!)^2 + 1 has been factored using ECM into P52*P96 with P52 = A083341(15). (61!)^2 + 1 is composite with 168 decimal digits.  Hugo Pfoertner, Jul 13 2019
Using CADONFS, (61!)^2 + 1 has been factored into P58*P110 with P58 = A282706(61) in 17 days wall clock time using 56 million CPU seconds. a(18) >= 75.  Hugo Pfoertner, Aug 04 2019


LINKS

Table of n, a(n) for n=1..17.
The CADONFS Development Team, CADONFS, An Implementation of the Number Field Sieve Algorithm, Release 2.3.0, 2017
Andrew Walker, Factors of (n!)^2+1.
factordb.com, Status of 75!^2+1.


EXAMPLE

a(1)=6 because (6!)^2+1=518401=13*39877 is a semiprime.


MATHEMATICA

Select[Range[60], PrimeOmega[(#!)^2+1]==2&] (* Harvey P. Dale, Dec 12 2018 *)


CROSSREFS

Cf. A020549, A001358, A083341, A046029, A282706.
Sequence in context: A274556 A066235 A037365 * A118733 A047275 A327301
Adjacent sequences: A083337 A083338 A083339 * A083341 A083342 A083343


KEYWORD

nonn,more,hard


AUTHOR

Hugo Pfoertner, Apr 24 2003


EXTENSIONS

a(15) from Hugo Pfoertner, Jul 13 2019
a(16), a(17) from Hugo Pfoertner, Aug 04 2019


STATUS

approved



