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A246198 Half-Zumkeller numbers: numbers n whose proper positive divisors can be partitioned into two disjoint sets whose sums are equal. 5
6, 12, 20, 24, 28, 30, 40, 42, 48, 54, 56, 60, 66, 70, 78, 80, 84, 88, 90, 96, 102, 104, 108, 112, 114, 120, 126, 132, 138, 140, 150, 156, 160, 168, 174, 176, 180, 186, 192, 198, 204, 208, 210, 216, 220, 222, 224, 225, 228, 234, 240, 246, 252, 258, 260, 264 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
All even half-Zumkeller numbers are in A083207, i.e. they are Zumkeller numbers (see Clark et al. 2008). The first 47 terms coincide with A083207. 225 is the first number in the sequence that is not a Zumkeller number.
REFERENCES
S. Clark et al., Zumkeller numbers, Mathematical Abundance Conference, April 2008.
LINKS
Robert Israel, Table of n, a(n) for n = 1..9188 (n=1..1309 from Chai Wah Wu)
K. P. S. Bhaskara Rao and Yuejian Peng, On Zumkeller Numbers, Journal of Number Theory, Volume 133, Issue 4, April 2013, pp. 1135-1155.
Pankaj Jyoti Mahanta, Manjil P. Saikia, and Daniel Yaqubi, Some properties of Zumkeller numbers and k-layered numbers, arXiv:2008.11096 [math.NT], 2020.
EXAMPLE
Proper divisors of 225 are 1, 3, 5, 9, 15, 25, 45, 75 and 1+3+15+25+45=5+9+75.
MAPLE
filter:= proc(n) local L, s, t, nL, B, j, k;
L:= numtheory:-divisors(n) minus {n};
s:= convert(L, `+`);
if s::odd then return false fi;
t:= s/2;
nL:= nops(L);
B:= Array(0..t, 1..nL);
B[0, 1]:= 1;
B[L[1], 1]:= 1;
for j from 2 to nL do
B[.., j]:= B[.., j-1];
for k from L[j] to t do
B[k, j]:= B[k, j] + B[k-L[j], j-1]
od:
if B[t, j] > 0 then return true fi;
od:
false
end:
select(filter, [$2..300]); # Robert Israel, Aug 19 2014
MATHEMATICA
filterQ[n_] := Module[{L, s, t, nL, B, j, k},
L = Most[Divisors[n]];
s = Total[L];
If[OddQ[s], Return[False]];
t = s/2;
nL = Length[L];
B[_, _] = 0;
B[0, 1] = 1;
B[L[[1]], 1] = 1;
For[j = 2, j <= nL, j++,
Do[B[k, j] = B[k, j-1], {k, 0, t}];
For[k = L[[j]], k <= t, k++,
B[k, j] = B[k, j] + B[k-L[[j]], j-1]
];
If[ B[t, j] > 0, Return[True]];
];
False
];
Select[Range[2, 300], filterQ] (* Jean-François Alcover, Mar 04 2019, after Robert Israel *)
hzQ[n_] := Module[{d = Most @ Divisors[n], sum, x}, sum = Plus @@ d; EvenQ[sum] && CoefficientList[Product[1 + x^i, {i, d}], x][[1 + sum/2]] > 0]; Select[Range[2, 1000], hzQ] (* Amiram Eldar, May 03 2020 *)
PROG
(Python)
from sympy.combinatorics.subsets import Subset
from sympy import divisors
A246198 = []
for n in range(2, 10**3):
....d = divisors(n)
....d.remove(n)
....s, dmax = sum(d), max(d)
....if not s % 2 and 2*dmax <= s:
........d.remove(dmax)
........s2 = s/2-dmax
........for x in range(2**len(d)):
............if sum(Subset.unrank_binary(x, d).subset) == s2:
................A246198.append(n)
................break
(Python)
from sympy import divisors
import numpy as np
A246198 = []
for n in range(2, 10**3):
....d = divisors(n)
....d.remove(n)
....s, dmax = sum(d), max(d)
....if not s % 2 and 2*dmax <= s:
........d.remove(dmax)
........s2, ld = int(s/2-dmax), len(d)
........z = np.zeros((ld+1, s2+1), dtype=int)
........for i in range(1, ld+1):
............y = min(d[i-1], s2+1)
............z[i, range(y)] = z[i-1, range(y)]
............z[i, range(y, s2+1)] = np.maximum(z[i-1, range(y, s2+1)], z[i-1, range(0, s2+1-y)]+y)
............if z[i, s2] == s2:
................A246198.append(n)
................break
# Chai Wah Wu, Aug 19 2014
CROSSREFS
Cf. A083207.
Sequence in context: A354931 A105455 A345919 * A083207 A370205 A304391
KEYWORD
nonn
AUTHOR
Chai Wah Wu, Aug 18 2014
STATUS
approved

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