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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.

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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

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]);