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A074837 Numbers n such that the penultimate 3 divisors of n sum to n. 2
6, 18, 42, 54, 66, 78, 102, 114, 126, 138, 162, 174, 186, 198, 222, 234, 246, 258, 282, 294, 306, 318, 342, 354, 366, 378, 402, 414, 426, 438, 462, 474, 486, 498, 522, 534, 546, 558, 582, 594, 606, 618, 642, 654, 666, 678, 702, 714, 726, 738, 762, 774, 786 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

It seems that only numbers that are 6 mod 12 are present except for multiples of 30.

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

18 has the divisors 1,2,3,6,9,18. The penultimate 3 are 3,6,9, which sum to 18.

MATHEMATICA

Select[Range[1000], Length[Divisors[ # ]]>3 && Sum[Divisors[ # ][[ -i]], {i, 2, 4}]==# &] (* Stefan Steinerberger, Aug 01 2007 *)

p3dQ[n_]:=Module[{d=Divisors[n]}, Length[d]>3&&Total[Take[Most[d], -3]] == n]; Select[Range[800], p3dQ] (* Harvey P. Dale, Dec 06 2012 *)

PROG

(PARI) for (n=1, 300, dn=divisors(n); dnl=length(dn); if (dnl>3, if (n==dn[dnl-1]+dn[dnl-2]+dn[dnl-3], print(n))))

CROSSREFS

Sequence in context: A163983 A191829 A023620 * A286308 A015942 A009945

Adjacent sequences:  A074834 A074835 A074836 * A074838 A074839 A074840

KEYWORD

nonn

AUTHOR

Jon Perry, Sep 09 2002

EXTENSIONS

More terms from Stefan Steinerberger, Aug 01 2007

STATUS

approved

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Last modified May 17 06:30 EDT 2021. Contains 343965 sequences. (Running on oeis4.)