login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A078778 Numbers n such that n!+1 is a semiprime. 8
4, 5, 6, 7, 8, 10, 13, 14, 19, 20, 24, 25, 26, 28, 34, 38, 48, 54, 55, 59, 71, 75, 92, 109, 114, 115 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Subsequence of (and likely is equal to) the union of A146968 and A181764. - Max Alekseyev, May 28 2015

Note that the two prime factors of 38!+1 = 523022617466601111760007224100074291200000001 = 14029308060317546154181 * 37280713718589679646221 both have 23 decimal digits. Are there any other terms in this sequence other than 4,5,7 and 38 with this property?

a(27) > 139. - Robert Price, Apr 11 2019

Other terms in this sequence: 392, 551, 601, 770, 772, 878, 1033, 1320, 1831, 2620, 2808, 3752, 4233, 4616, 4984, 7260. - Chai Wah Wu, Feb 28 2020

LINKS

Table of n, a(n) for n=1..26.

FactorDB, Status of 140!+1.

EXAMPLE

4 is in the sequence because 4!+1=25=5*5 is semiprime. But 9 is not in the sequence because 9!+1=19*71*269 is not semiprime. - Sean A. Irvine, Nov 15 2009

MATHEMATICA

Select[Range[100], Plus@@Last/@FactorInteger[#! + 1]==2 &] (* Vincenzo Librandi, May 26 2015 *)

Select[Range[100], PrimeOmega[#!+1]==2&] (* Harvey P. Dale, Mar 19 2017 *)

PROG

(PARI) { fp(a, b)=local(c, d, r); for(n=a, b, r=n!+1; c=vecmin(factor(r)[, 1]~); d=vecmax(factor(r)[, 1]~); if(bigomega(r)==2 && isprime(c) && isprime(d), print1(n" "); )) } fp(1, 100)

(MAGMA) IsSemiprime:=func< n | &+[ k[2]: k in Factorization(n) ] eq 2 >; [n: n in [1..60] | IsSemiprime(Factorial(n)+1)]; // Vincenzo Librandi, May 26 2015

CROSSREFS

Cf. A001358, A082952, A090159, A090160, A078781.

Sequence in context: A121541 A182305 A023373 * A037356 A139199 A003666

Adjacent sequences:  A078775 A078776 A078777 * A078779 A078780 A078781

KEYWORD

more,nonn

AUTHOR

Jason Earls, Jan 09 2003

EXTENSIONS

Term 109 from Sean A. Irvine, Nov 15 2009

Term 114 (factored by Womack et al.) from Sean A. Irvine, May 25 2015

Term 115 (factored by Womack et al.) from Sean A. Irvine, Feb 08 2016

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified February 28 09:24 EST 2021. Contains 341695 sequences. (Running on oeis4.)