A328416 - OEIS

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A328416

Smallest k such that (Z/mZ)* = C_2 X C_(2k) has exactly n solutions for m, or 0 if no such k exists, where (Z/mZ)* is the multiplicative group of integers modulo m.

7, 4, 1, 5, 2, 10, 21, 6, 42, 90, 150, 30, 78, 210, 2730, 690, 1050

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OFFSET

0,1

COMMENTS

Conjecture: a(n) > 0 for all n. That is to say, every number occurs in A328412.

It seems that most terms are congruent to 2 modulo 4.

LINKS

Table of n, a(n) for n=0..16.

Wikipedia, Multiplicative group of integers modulo n

EXAMPLE

(Z/mZ)* = C_2 X C_42 has exactly 6 solutions m = 129, 147, 172, 196, 258, 294; for any k < 21, (Z/mZ)* = C_2 X C_(2k) has either fewer than or more than 6 solutions, so a(6) = 21.

PROG

(PARI) a(n) = my(k=1); while(A328412(k)!=n, k++); k \\ See A328412 for its program

CROSSREFS

Cf. A328412.

Sequence in context: A010138 A371851 A199157 * A372275 A373809 A010508

Adjacent sequences: A328413 A328414 A328415 * A328417 A328418 A328419

KEYWORD

nonn,hard,more

AUTHOR

Jianing Song, Oct 14 2019

STATUS

approved