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A270417
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Number of integer-sided right triangles with semiperimeter n.
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1
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0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 2, 0, 0, 0, 0, 1, 1, 0, 0, 0, 1, 0, 2, 0, 0, 2, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 3, 0, 0, 1, 0, 0, 2, 0, 0, 0, 1, 0, 2, 0, 0, 1, 0, 1, 1, 0, 1, 0, 0, 0, 3, 0, 0, 0, 1, 0, 3, 1, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 1, 0, 1, 2, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 1
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OFFSET
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1,30
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COMMENTS
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Number of positive integer solutions to x*y*(y+z) = n with y and z coprime and of opposite parity and z < y.
Records occur at 1, 6, 30, 60, 120, 210, 360, 420, 840, 1260, 2310, 2520, 4620, 9240, 13860, 27720, 55440, 60060, ... - Antti Karttunen, Sep 25 2018
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LINKS
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EXAMPLE
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a(25)=0 since 2*25 = 50 is not the perimeter of a suitable triangle;
a(30)=2 since 2*30 = 60 = 15+20+25 = 10+24+26;
a(35)=1 since 2*35 = 70 = 20+21+29.
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MATHEMATICA
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a[n_] := Count[{x, y, z} /. {ToRules[Reduce[x>0 && y>0 && z>0 && z<y && x*y*(y+z) == n, {x, y, z}, Integers]]}, {x_, y_, z_} /; CoprimeQ[y, z] && Not[OddQ[y] && OddQ[z] || EvenQ[y] && EvenQ[z]]]; Array[a, 117] (* Jean-François Alcover, Jun 03 2017 *)
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PROG
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(PARI) A270417(n) = { my(s=0); fordiv(n, x, fordiv(n/x, y, my(w=n/(x*y)); if((w < 2*y)&&(w>y)&&(w%2)&&(1==gcd(w, y)), s++))); (s); }; \\ (Here z = w-y) - Antti Karttunen, Sep 25 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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