OFFSET
2,2
COMMENTS
a(n) is the first k such that the smallest m such that C_(2k) is a subgroup of (Z/mZ)* is m = prime(n), where C_(2k) is the cyclic group of order 2k and (Z/mZ)* is the multiplicative group of integers modulo m.
a(n) is well-defined since A307437((p-1)/2) = p for odd primes p.
LINKS
Jianing Song, Table of n, a(n) for n = 2..500
EXAMPLE
For n = 7, prime(n) = 17. The first k such that: (i) C_(2k) is a subgroup of (Z/17Z)*; (ii) there is no m < 17 such that C_(2k) is a subgroup of (Z/mZ)* is k = 4, so a(7) = 4.
For n = 21, prime(n) = 73. The first k such that: (i) C_(2k) is a subgroup of (Z/73Z)*; (ii) there is no m < 73 such that C_(2k) is a subgroup of (Z/mZ)* is k = 12, so a(21) = 12.
PROG
CROSSREFS
KEYWORD
nonn
AUTHOR
Jianing Song, Feb 26 2021
STATUS
approved