mersenneforum.org (https://www.mersenneforum.org/index.php)
-   enzocreti (https://www.mersenneforum.org/forumdisplay.php?f=156)
-   -   ...factorial(n) plus/minus 1 is semiprime (https://www.mersenneforum.org/showthread.php?t=25894)

 enzocreti 2020-09-01 08:43

...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

 Dylan14 2020-09-01 17:13

This would be the intersection of [OEIS]078778[/OEIS] (n such that n!+1 is semiprime) and [OEIS]078781[/OEIS] (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=enzocreti;555623]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[/QUOTE]

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

 All times are UTC. The time now is 09:47.