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A005793
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Number of O_1^{2+}(Z)-orbits of Lorentzian modular group.
(Formerly M0078)
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4
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1, 1, 1, 2, 1, 1, 2, 3, 2, 1, 2, 3, 1, 2, 3, 4, 2, 2, 2, 4, 2, 1, 4, 5, 2, 2, 3, 4, 2, 2, 4, 6, 2, 2, 4, 6, 1, 2, 5, 5, 3, 2, 2, 6, 3, 2, 6, 7, 3, 3, 4, 4, 2, 3, 5, 8, 2, 1, 5, 7, 2, 3, 7, 7, 3, 3, 2, 7, 3, 2, 8, 8, 2, 3, 5, 6, 3, 2, 6, 10, 4, 2, 5, 8, 2, 3, 7, 5, 4, 4, 4, 8, 2, 3, 9, 11, 2, 4, 6, 6, 4, 2, 6, 10
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OFFSET
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1,4
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COMMENTS
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Table 2 of Fox and Parker lists a(n) for n=1 to 1000. - Michael Somos, Jul 13 2013
"Each O_1^{2+}(Z)-orbit has a representative (z, x, y) in Z^3 with z > x >= y >= 0 and z >= x+y" from proof of Corollary 2.4 of Fox and Parker. We are looking for solutions of n = z^2 - x^2 - y^2. - Michael Somos, Jul 13 2013
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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EXAMPLE
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x + x^2 + x^3 + 2*x^4 + x^5 + x^6 + 2*x^7 + 3*x^8 + 2*x^9 + x^10 + 3*x^11
+ ...
a(8) = 3 since orbits(8) = [[3, 1, 0], [4, 2, 2], [5, 4, 1]] where 8 = 3^2-1^2-0^2 = 4^2-2^2-2^2 = 5^2-4^2-1^2 for the three O_1^{2+}(Z)-orbit representatives. - Michael Somos, Jul 13 2013
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MATHEMATICA
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r[n_] := Reduce[z > x >= y >= 0 && z >= x + y && n == z^2 - x^2 - y^2, {x, y, z}, Integers]; a[n_] := Switch[rn = r[n]; Head[rn], And, 1, Or, Length[rn]]; Table[Print["a(", n, ") = ", an = a[n], " ", {x, y, z} /. {ToRules[rn]}]; an, {n, 1, 105}]
(* or, from 1st PARI script: *) a[n_] := Sum[If[Mod[n - i, 2] != 0, 0, DivisorSum[j = (n + i*i)/2, Boole[# >= i && #^2 <= j] &]], {i, 0, Sqrt[n] }]; Array[a, 105] (* Jean-François Alcover, Dec 02 2015 *)
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PROG
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(PARI) {a(n) = my(j); if( n<1, 0, sum( i=0, sqrtint(n), if( (n-i)%2, 0, sumdiv( j = (n + i*i) / 2, d, d>=i && d*d <= j))))} /* Michael Somos, Feb 14 2002 */
(PARI) {orbits(n) = local(j, v=[]); if( n<1, 0, forstep( i=n%2, sqrtint(n), 2, fordiv( j = (n + i*i) / 2, d, if( d>=i && d*d <= j, v = concat( [[d+j/d-i, j/d-i, d-i]], v)))); vecsort(v))} /* Michael Somos, Jul 13 2013 */
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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