A087247 - OEIS

A087247

Squarefree deficient nonprime numbers.

1, 10, 14, 15, 21, 22, 26, 33, 34, 35, 38, 39, 46, 51, 55, 57, 58, 62, 65, 69, 74, 77, 82, 85, 86, 87, 91, 93, 94, 95, 105, 106, 110, 111, 115, 118, 119, 122, 123, 129, 130, 133, 134, 141, 142, 143, 145, 146, 154, 155, 158, 159, 161, 165, 166, 170, 177, 178, 182, 183

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

OFFSET

1,2

LINKS

Enrique Pérez Herrero, Table of n, a(n) for n = 1..10000

EXAMPLE

105 = 3*5*7 and sigma(105) = 1 + 3 + 5 + 7 + 15 + 21 + 35 + 105 = 192 < 210 = 2*105, so 105 is in the sequence.

The sequence differs from A006881: first term with 3 distinct prime factors is 105.

MATHEMATICA

isA087247[n_] := SquareFreeQ[n] && !PrimeQ[n] && (DivisorSigma[1, n] < 2n); Select[Range[200], isA087247] (* Enrique Pérez Herrero, Jan 13 2011 *)

PROG

(PARI) isok(n) = !isprime(n) && issquarefree(n) && (sigma(n) < 2*n); \\ Michel Marcus, Jul 09 2018

CROSSREFS

Cf. A087244, A087245, A087246, A087248.

Cf. A008683, A000203, A013929, A005117, A005100, A005101.

Sequence in context: A307980 A366413 A367018 * A307507 A171156 A235690

Adjacent sequences: A087244 A087245 A087246 * A087248 A087249 A087250

KEYWORD

nonn

AUTHOR

Labos Elemer, Sep 05 2003

STATUS

approved