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URL: https://oeis.org/A390312

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A390312
Numbers k such that sigma(k) has at least one prime factor larger than the largest prime factor of k.
1
2, 4, 8, 9, 12, 16, 18, 20, 24, 25, 27, 32, 36, 45, 48, 49, 50, 54, 60, 63, 64, 72, 75, 80, 81, 90, 96, 98, 99, 100, 108, 112, 121, 125, 126, 128, 144, 147, 150, 160, 162, 169, 175, 176, 180, 192, 196, 198, 200, 208, 216, 225, 240, 242, 243, 245, 250, 252, 256, 272, 275, 288, 289, 294, 300, 304
OFFSET
1,1
LINKS
EXAMPLE
a(3) = 8 is a term because sigma(8) = 15 has prime factor 5, while the largest prime factor of 8 is 2.
MAPLE
filter:= proc(k) uses NumberTheory;
max(PrimeFactors(sigma(k))) > max(PrimeFactors(k));
end proc:
select(filter, [$1..400]);
MATHEMATICA
Select[Range[310], Max[First/@FactorInteger[DivisorSigma[1, #]]]>Max[First/@FactorInteger[#]] &] (* Stefano Spezia, Dec 29 2025 *)
CROSSREFS
Complement of A336353. Cf. A102750, A336317, A336352.
Sequence in context: A304451 A098374 A182852 * A380743 A318083 A033858
KEYWORD
nonn
AUTHOR
Robert Israel, Dec 29 2025
STATUS
approved