...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

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).

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

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

[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