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A098594 Numbers n such that n!-1 and n!+1 are both semiprime. 0
5, 8, 10, 13, 20, 24, 26, 34, 59, 392 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This sequence is formed of all those terms that appear in both A078778 and A078781.

No more terms < 307. - David Wasserman, Feb 26 2008

a(11) >= 464. - D. S. McNeil, Sep 04 2011

a(11) >= 929. 929!-1 is semiprime, no factor of 929!+1 is known. - Sean A. Irvine, Mar 09 2013

LINKS

Table of n, a(n) for n=1..10.

EXAMPLE

10!+1 = 3628801 = 11*329891 and 10!-1 = 3628799 = 29*125131 so 10 is a member of the sequence.

464 is not a term since 464!-1=2828197538205421590987128183441789966021011*C996 is not a semiprime. - Sean A. Irvine, Mar 09 2013

MAPLE

out:=[]: for n from 1 to 60 do: a:=n!-1: b:=n!+1: if (bigomega(a)=2) and (bigomega(b)=2) then out:=[op(out), n]: print(n): fi: od: out;

CROSSREFS

Cf. A078778, A078781.

Sequence in context: A032717 A256360 A205676 * A154315 A182084 A262707

Adjacent sequences:  A098591 A098592 A098593 * A098595 A098596 A098597

KEYWORD

hard,more,nonn

AUTHOR

Mark Hudson (mrmarkhudson(AT)hotmail.com), Sep 17 2004

EXTENSIONS

a(10) from D. S. McNeil, Sep 04 2011

STATUS

approved

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Last modified October 23 12:10 EDT 2018. Contains 316527 sequences. (Running on oeis4.)