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 A328149 Numbers whose set of divisors contains a quadruple (x, y, z, w) satisfying x^3 + y^3 + z^3 = w^3. 4
 60, 72, 120, 144, 180, 216, 240, 288, 300, 360, 420, 432, 480, 504, 540, 576, 600, 648, 660, 720, 780, 792, 840, 864, 900, 936, 960, 1008, 1020, 1080, 1140, 1152, 1200, 1224, 1260, 1296, 1320, 1368, 1380, 1440, 1500, 1512, 1560, 1584, 1620, 1656, 1680, 1710 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The subsequence of numbers of the form 2^i*3^j is 72, 144, 216, 288, 432, 576, 648, 864, 1152, 1296, ... The corresponding number of quadruples of the sequence is 1, 1, 2, 2, 2, 2, 3, 3, 2, 6, 2, 4, 4, 2, 3, 4, 4, 3, 2, 10, ... (see the sequence A328204). The set of divisors of a(n) contains at least one primitive quadruple. Examples:   The set of divisors of a(1) = 60 contains only one primitive quadruple: (3, 4, 5, 6).   The set of divisors of a(10) = 360 contains two primitive quadruples: (1, 6, 8, 9) and (3, 4, 5, 6). From Robert Israel, Jul 06 2020: (Start) Every multiple of a member of the sequence is in the sequence. The first member of the sequence not divisible by 6 is a(68) = 2380, which has the quadruple (7, 14, 17, 20). The first odd member of the sequence is a(1230) = 43065, which has the quadruple (11, 15, 27, 29). (End) REFERENCES Y. Perelman, Solutions to x^3 + y^3 + z^3 = u^3, Mathematics can be Fun, pp. 316-9 Mir Moscow 1985. LINKS Robert Israel, Table of n, a(n) for n = 1..10000 Fred Richman, Sums of Three Cubes EXAMPLE 120 is in the sequence because the set of divisors {1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, 120} contains the quadruples {3, 4, 5, 6} and {6, 8, 10, 12}. The first quadruple is primitive. MAPLE with(numtheory): for n from 3 to 2000 do :    d:=divisors(n):n0:=nops(d):it:=0:     for i from 1 to n0-3 do:      for j from i+1 to n0-2 do :       for k from j+1 to n0-1 do:       for m from k+1 to n0 do:        if d[i]^3 + d[j]^3 + d[k]^3 = d[m]^3         then         it:=it+1:         else        fi:       od:      od:     od:     od:     if it>0 then     printf(`%d, `, n):     else fi:    od: MATHEMATICA nq[n_] := If[ Mod[n, 6]>0, 0, Block[{t, u, v, c = 0, d = Divisors[n], m}, m = Length@ d; Do[ t = d[[i]]^3 + d[[j]]^3; Do[u = t + d[[h]]^3; If[u > n^3, Break[]]; If[ Mod[n^3, u] == 0 && IntegerQ[v = u^(1/3)] && Mod[n, v] == 0, c++], {h, j+1, m - 1}], {i, m-3}, {j, i+1, m - 2}]; c]]; Select[ Range@ 1026, nq[#] > 0 &] (* program from Giovanni Resta adapted for the sequence. See A330893 *) PROG (PARI) isok(n) = {my(d=divisors(n), m); if (#d > 3, for (i=1, #d-3, for (j=i+1, #d-2, for (k=j+1, #d-1, if (ispower(d[i]^3+d[j]^3+d[k]^3, 3, &m) && !(n%m), return (1)); ); ); ); ); } \\ Michel Marcus, Nov 15 2020 CROSSREFS Cf. A023042, A027750, A096545, A328204, A330893. Sequence in context: A347935 A347938 A114837 * A154548 A244384 A290018 Adjacent sequences:  A328146 A328147 A328148 * A328150 A328151 A328152 KEYWORD nonn AUTHOR Michel Lagneau, Jun 07 2020 STATUS approved

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Last modified October 23 14:42 EDT 2021. Contains 348214 sequences. (Running on oeis4.)