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A030513 Numbers with 4 divisors. 31
6, 8, 10, 14, 15, 21, 22, 26, 27, 33, 34, 35, 38, 39, 46, 51, 55, 57, 58, 62, 65, 69, 74, 77, 82, 85, 86, 87, 91, 93, 94, 95, 106, 111, 115, 118, 119, 122, 123, 125, 129, 133, 134, 141, 142, 143, 145, 146, 155, 158, 159, 161, 166, 177, 178, 183, 185, 187 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Essentially the same as A007422.

Numbers which are either the product of two distinct primes (A006881) or the cube of a prime (A030078).

4*a(n) are the solutions to A048272(x) = Sum{d|x} (-1)^d = 4. - Benoit Cloitre, Apr 14 2002

Since A119479(4)=3, there are never more than 3 consecutive terms. Triples of consecutive terms start at 33, 85, 93, 141, 201... (A039833). No such triple contains a term of the form p^3. - Ivan Neretin, Feb 08 2016

LINKS

R. J. Mathar, Table of n, a(n) for n = 1..1000

R. J. Mathar, Maple programs for A030638, A030637, A030636, A030635, A030634, A030633, A030632, A030631, A030630, A030629, A030628, A030627, A030626, A030516, A030515, A030514, A030513

FORMULA

{n : A000005(n) = 4}. - Juri-Stepan Gerasimov, Oct 10 2009

MAPLE

A030513=proc(q) local n;

     for n from 1 to q do if tau(n)=4 then print(n); fi; od;

end:

A030513(10^10); # Paolo P. Lava, Mar 18 2013

MATHEMATICA

Select[Range[200], DivisorSigma[0, #]==4&] (* Harvey P. Dale, Apr 06 2011 *)

PROG

(PARI) is(n)=numdiv(n)==4 \\ Charles R Greathouse IV, May 18 2015

(MAGMA) [n: n in [1..200] | DivisorSigma(0, n) eq 4]; // Vincenzo Librandi, Jul 16 2015

CROSSREFS

Cf. A000005, A006881, A007422, A030078, A030515, A035533, A048272.

Sequence in context: A291127 A211337 A007422 * A161918 A294729 A242270

Adjacent sequences:  A030510 A030511 A030512 * A030514 A030515 A030516

KEYWORD

nonn,easy,nice

AUTHOR

Jeff Burch

EXTENSIONS

Incorrect comments removed by Charles R Greathouse IV, Mar 18 2010

STATUS

approved

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Last modified October 15 21:17 EDT 2019. Contains 328038 sequences. (Running on oeis4.)