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A342038
a(n) is the index of the first occurrence of prime(n) in A307437.
2
1, 2, 3, 5, 6, 4, 9, 11, 7, 15, 18, 20, 21, 23, 13, 29, 30, 33, 35, 12, 39, 41, 22, 16, 25, 17, 53, 54, 28, 63, 65, 34, 69, 37, 75, 78, 81, 83, 43, 89, 45, 19, 32, 49, 99, 105, 111, 113, 38, 58, 119, 60, 125, 64, 131, 67, 135, 138, 70, 47, 73, 153, 31, 52, 79
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
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
(PARI) a(n) = if(n>=2, my(p=prime(n)); for(k=1, oo, if(A307437(k)==p, return(k)))) \\ see A307437 for its program
CROSSREFS
Sequence in context: A194900 A194051 A195610 * A082654 A072636 A191741
KEYWORD
nonn
AUTHOR
Jianing Song, Feb 26 2021
STATUS
approved