OFFSET
1,3
COMMENTS
This map is used in A339991.
MATHEMATICA
Array[Which[EvenQ@ #, #/2, PrimeQ@ #, #^2 - 1, True, # - 1] &, 64] (* Michael De Vlieger, Dec 28 2020 *)
PROG
(Python)
from sympy import isprime
for n in range(1, 1001):
if n%2 == 0: a = n/2
elif isprime(n) == 1: a = n*n - 1
else: a = n - 1
print(a)
(PARI) a(n) = if (n%2, if (isprime(n), n^2-1, n-1), n/2); \\ Michel Marcus, Dec 26 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Ya-Ping Lu, Dec 26 2020
STATUS
approved