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A175388 Numbers n such that the sum of the divisors of n is semiprime. 1
3, 5, 8, 13, 18, 36, 37, 49, 50, 61, 73, 81, 100, 121, 144, 157, 169, 193, 225, 256, 277, 313, 361, 397, 400, 421, 457, 529, 541, 576, 578, 613, 625, 661, 673, 733, 757, 841, 877, 961, 997, 1024, 1093, 1153, 1156, 1201, 1213, 1237, 1321, 1381, 1453 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The sequence contains a subset of squares {36, 49, 81, 100, 121, 144, 169, 225, 256, ...}.

If n is a term of this sequence and n is nonsquare, then n must be a prime or twice a square. Additionally, if n is in this sequence, then A001221(n) <= 2. - Altug Alkan, Jul 17 2016

LINKS

G. C. Greubel, Table of n, a(n) for n = 1..1000

EXAMPLE

a(6)= 36 with 8 divisors: {1, 2, 3, 4, 6, 9, 12, 18, 36}

and the sum of the divisors is 91 = 7*13 (semiprime).

MAPLE

with(numtheory):for k from 1 to 1600 do:if bigomega(sigma(k))=2  then printf(`%d, `, k): else fi:od:

MATHEMATICA

Select[Range@ 1500, PrimeOmega@ DivisorSigma[1, #] == 2 &] (* Michael De Vlieger, Jul 17 2016 *)

CROSSREFS

Cf. A112381, A023194 (numbers n such that sigma(n) is prime).

Sequence in context: A265046 A158384 A053651 * A104563 A261175 A265064

Adjacent sequences:  A175385 A175386 A175387 * A175389 A175390 A175391

KEYWORD

nonn

AUTHOR

Michel Lagneau, Aug 20 2011

STATUS

approved

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Last modified November 22 08:57 EST 2017. Contains 295076 sequences.