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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 Adjacent sequences: A246195 A246196 A246197 * A246199 A246200 A246201 KEYWORD nonn AUTHOR Chai Wah Wu, Aug 18 2014 STATUS approved

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