...factorial(n) plus/minus 1 is semiprime

enzocreti · 2020-09-01, 08:43

Is there an Oeis sequence dedicated to numbers n such that n!+1 and n!-1 are both semiprime? An example is 20970

Infact 20970!+1 is semiprime and also 20970!-1 is semi prime

No the example is wrong

Dylan14 · 2020-09-01, 17:13

This would be the intersection of 078778 (n such that n!+1 is semiprime) and 078781 (n such that n!-1 is semiprime). There is no separate sequence (yet, as far as I can tell).

CRGreathouse · 2020-09-02, 03:32

The first few terms are: 5, 8, 10, 13, 20, 24, 26, 34, 59, 392.

392! - 1 = 78958785391722157 * P832.
392! + 1 = 32749 * P844.

sweety439 · 2020-09-02, 16:35

Quote:

Originally Posted by enzocreti

Is there an Oeis sequence dedicated to numbers n such that n!+1 and n!-1 are both semiprime? An example is 20970

Infact 20970!+1 is semiprime and also 20970!-1 is semi prime

No the example is wrong

Neither 20970!+1 nor 20970!-1 is semiprime, 20970!+1 = 7778806327 * C81510, and 20970!-1 = 4156001 * C81513

2020-09-01, 08:43	#1
enzocreti Mar 2018 21B₁₆ Posts	...factorial(n) plus/minus 1 is semiprime Is there an Oeis sequence dedicated to numbers n such that n!+1 and n!-1 are both semiprime? An example is 20970 Infact 20970!+1 is semiprime and also 20970!-1 is semi prime No the example is wrong Last fiddled with by enzocreti on 2020-09-01 at 08:46

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Semiprime and n-almost prime candidate for the k's with algebra for the Sierpinski/Riesel problem	sweety439	sweety439	11	2020-09-23 01:42
Semiprime counting	danaj	Computer Science & Computational Number Theory	12	2018-10-14 03:16
Generating a uniform random n-bit semiprime	danaj	Programming	17	2017-09-15 16:41
Factored my first SemiPrime -Some Questions	tal	Msieve	31	2011-05-22 18:17
2 minus table: 2,781-	R.D. Silverman	NFSNET Discussion	3	2006-01-16 00:48

2020-09-01, 17:13	#2
Dylan14 "Dylan" Mar 2017 2·3³·11 Posts	This would be the intersection of 078778 (n such that n!+1 is semiprime) and 078781 (n such that n!-1 is semiprime). There is no separate sequence (yet, as far as I can tell).

2020-09-02, 03:32	#3
CRGreathouse Aug 2006 1011101100011₂ Posts	The first few terms are: 5, 8, 10, 13, 20, 24, 26, 34, 59, 392. 392! - 1 = 78958785391722157 * P832. 392! + 1 = 32749 * P844.