A202707 - OEIS

A202707

Numbers k such that (sum of digits of k!!) / 9 is prime.

9, 11, 12, 13, 14, 15, 16, 18, 19, 21, 22, 25, 26, 28, 48, 51, 57, 60, 70, 72, 74, 78, 80, 81, 82, 86, 89, 92, 103, 109, 111, 114, 120, 125, 128, 130, 131, 142, 145, 146, 151, 155, 157, 159, 164, 168, 169, 179, 183, 185, 186, 191, 195, 197, 200, 205, 210, 211

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

Numbers k such that A120390(k) is prime.

If k = 9 or k > 10, then (sum of digits of k!!) / 9 is an integer (see A120390).

LINKS

Table of n, a(n) for n=1..58.

EXAMPLE

For k = 9, 9!! = 945, and (9+4+5)/9 = 2 is prime.

MATHEMATICA

lst={}; Do[If[PrimeQ[Sum[DigitCount[n!!][[i]]*i/9, {i, 1, 9}]], AppendTo[lst, n]], {n, 1, 300}]; lst