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A204831 Numbers n whose divisors can be partitioned into four disjoint sets whose sums are all sigma(n)/4. 7
27720, 30240, 32760, 50400, 55440, 60480, 65520, 75600, 83160, 85680, 90720, 95760, 98280, 100800, 105840, 110880, 115920, 120120, 120960, 128520, 131040, 138600, 143640, 151200, 163800, 166320, 171360, 180180, 181440, 184800, 191520 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Subsequence of A023198 (numbers n such that sigma(n) >= 4n).

LINKS

Table of n, a(n) for n=1..31.

EXAMPLE

Number 27720 is in the sequence because sigma(27720)/4 = 28080 = 360 + 27720 = 20 + 60 + 280 + 2310 + 4620 + 6930 + 13860 = 9 + 30 + 420 + 1540 + 1980 + 2772 + 3080 + 3465 + 5544 + 9240 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 10 + 11 + 12 + 14 + 15 + 18 + 21 + 22 + 24 + 28 + 33 + 35 + 36 + 40 + 42 + 44 + 45 + 55 + 56 + 63 + 66 + 70+ 72 + 77 + 84 + 88 + 90 + 99 + 105 + 110 + 120 + 126 + 132 + 140 + 154 + 165 + 168 + 180 + 198 + 210 + 220 + 231 + 252 + 264+ 308 + 315 + 330 + 385 + 396 + 440 + 462 + 495 + 504 + 616 + 630 + 660 + 693 + 770 + 792 + 840 + 924 + 990 + 1155 + 1260 + 1320 + 1386 + 1848 + 2520 + 3960 (summands are all divisors of 27720).

MAPLE

with(numtheory); with(combstruct);

A204831:=proc(i)

local S, R, Stop, Comb, c, d, k, m, n, s;

for n from 1 to i do

  s:=sigma(n); c:=op(divisors(n));

  if (modp(s, 4)=0 and 4*n<=s) then

     S:=1/4*s-n; R:=select(m->m<=S, [c]); Stop:=false;

     Comb:=iterstructs(Combination(R));

     while not (finished(Comb) or Stop) do

       Stop:=add(d, d=nextstruct(Comb))=S;

     od;

     if Stop then print(n); fi;

  fi;

od;

end:

A204831(100000); # Paolo P. Lava, Jan 24 2012

CROSSREFS

Cf. A083207 (Zumkeller numbers--numbers n whose divisors can be partitioned into two disjoint sets whose sums are both sigma(n)/2), A204830 (numbers n whose divisors can be partitioned into three disjoint sets whose sums are all sigma(n)/3).

Sequence in context: A251235 A023198 A230608 * A190111 A068404 A279091

Adjacent sequences:  A204828 A204829 A204830 * A204832 A204833 A204834

KEYWORD

nonn

AUTHOR

Jaroslav Krizek, Jan 22 2012

STATUS

approved

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Last modified December 15 09:05 EST 2019. Contains 329995 sequences. (Running on oeis4.)