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A308098
Numbers m such that sequence of their values of sigma(m) corresponds to sequence of unique values of function sigma(n) for n >= 1 in increasing order (A007370).
1
1, 2, 3, 5, 4, 7, 9, 13, 8, 19, 12, 29, 22, 37, 18, 27, 43, 49, 61, 32, 67, 73, 45, 36, 50, 101, 109, 91, 81, 64, 121, 137, 149, 157, 133, 106, 163, 98, 173, 129, 169, 193, 72, 197, 199, 134, 211, 100, 146, 229, 241, 128, 217, 257, 171, 148, 277, 281, 283, 219
OFFSET
1,2
COMMENTS
A211656 is the sorted version of this sequence.
FORMULA
A000203(a(n)) = A007370(n) for all n.
EXAMPLE
a(6) = 7 because A007370(6) = 8 and there is only one solution of equation sigma(x) = 8 for x = 7.
MATHEMATICA
m = 500; v = Table[0, {m}]; Do[s = DivisorSigma[1, k]; If[s <= m , v[[s]] = If[ v[[s]] == 0, k, -1]], {k, 1, m - 1}]; Select[v, # > 0 &] (* Amiram Eldar, Jul 04 2019 *)
PROG
(Magma) [[m: m in [1..1000] | SumOfDivisors(m) eq n]: n in [1..100] | #[#[m]: m in [1..1000] | SumOfDivisors(m) eq n] eq 1]
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, May 12 2019
STATUS
approved