A230306 - OEIS

A230306

Evil numbers (A001969) which can be written as a product of two evil numbers > 1.

9, 15, 18, 27, 30, 36, 45, 51, 54, 60, 72, 75, 85, 90, 99, 102, 108, 120, 129, 135, 144, 150, 153, 159, 165, 170, 180, 189, 195, 198, 204, 207, 215, 216, 225, 231, 240, 243, 249, 255, 258, 267, 270, 288, 297, 300, 303, 306, 315, 318, 325, 330, 340, 360, 378

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

OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

EXAMPLE

Evil number 75 = 3*5*5. Since 3 and 5 are evil numbers, than 75 is in the sequence.

MATHEMATICA

evilQ[n_] := EvenQ[DigitCount[n, 2][[1]]]; fQ[n_] := Module[{f, i}, If[PrimeQ[n], False, f = Select[Divisors[n], # > 1 && # <= Sqrt[n] &]; i = 1; While[i <= Length[f] && ! (evilQ[f[[i]]] && evilQ[n/f[[i]]]), i++]; i <= Length[f]]]; Select[Range[1000], evilQ[#] && fQ[#] &] (* T. D. Noe, Oct 16 2013 *)

CROSSREFS

Cf. A001969, A227932, A230213, A230226.

Sequence in context: A177733 A207675 A118236 * A070876 A266419 A161163

Adjacent sequences: A230303 A230304 A230305 * A230307 A230308 A230309

KEYWORD

nonn,base

AUTHOR

Vladimir Shevelev, Oct 15 2013

EXTENSIONS

Extended by T. D. Noe, Oct 16 2013

a(1)=0 removed by Amiram Eldar, Sep 23 2019

STATUS

approved