A340008 - OEIS

%I #15 Jan 05 2021 21:44:51

%S 0,1,8,2,24,3,48,4,8,5,120,6,168,7,14,8,288,9,360,10,20,11,528,12,24,

%T 13,26,14,840,15,960,16,32,17,34,18,1368,19,38,20,1680,21,1848,22,44,

%U 23,2208,24,48,25,50,26,2808,27,54,28,56,29,3480,30,3720,31,62,32

%N a(n) is the image of n under the map: n -> n/2 if n is even, n-> n^2 - 1 if n is an odd prime, otherwise n -> n - 1.

%C This map is used in A339991.

%t Array[Which[EvenQ@ #, #/2, PrimeQ@ #, #^2 - 1, True, # - 1] &, 64] (* _Michael De Vlieger_, Dec 28 2020 *)

%o (Python)

%o from sympy import isprime

%o for n in range(1, 1001):

%o     if n%2 == 0: a = n/2

%o     elif isprime(n) == 1: a = n*n - 1

%o     else: a = n - 1

%o     print(a)

%o (PARI) a(n) = if (n%2, if (isprime(n), n^2-1, n-1), n/2); \\ _Michel Marcus_, Dec 26 2020

%Y Cf. A339991.

%K nonn

%O 1,3

%A _Ya-Ping Lu_, Dec 26 2020