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A001767
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Genus of modular group Gamma(n) = genus of modular curve Chi(n).
(Formerly M2459 N0976)
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1
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0, 0, 0, 0, 1, 3, 5, 10, 13, 26, 25, 50, 49, 73, 81, 133, 109, 196, 169, 241, 241, 375, 289, 476, 421, 568, 529, 806, 577, 1001, 833, 1081, 1009, 1393, 1081, 1768, 1441, 1849, 1633, 2451, 1729, 2850, 2281, 2809, 2641, 3773, 2689, 4215, 3301, 4321, 3865, 5500
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OFFSET
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2,6
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COMMENTS
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In Klein and Fricke, the level n is called Stufenzahlen, the congruence group is denoted by Gamma_{n} and the genus is called Geschlecht and denoted by p. - Michael Somos, Nov 08 2014
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REFERENCES
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R. C. Gunning, Lectures on Modular Forms. Princeton Univ. Press, Princeton, NJ, 1962, p. 15.
B. Iversen, Hyperbolic Geometry, Cambridge Univ. Press, 1992, see p. 238.
F. Klein and R. Fricke, Vorlesungen ueber die theorie der elliptischen modulfunctionen, Teubner, Leipzig, 1890, Vol. 1, see p. 398.
Russian Encyclopedia of Mathematics, Vol. 3, page 931.
B. Schoeneberg, Elliptic Modular Functions, Springer-Verlag, NY, 1974, p. 94.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
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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FORMULA
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EXAMPLE
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G.f. = x^6 + 3*x^7 + 5*x^8 + 10*x^9 + 13*x^10 + 26*x^11 + 25*x^12 + ...
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MATHEMATICA
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Join[{0}, Table[1 + n^2 (n - 6)/24 Product[If[Mod[n, Prime[p]] == 0, 1 - 1/Prime[p]^2, 1], {p, PrimePi[n]}], {n, 3, 100}]] (* T. D. Noe, Aug 10 2012 *)
a[ n_] := If[ n < 3, 0, 1 + n^2 (n - 6)/24 Product[ If[ PrimeQ[p] && Divisible[n, p], 1 - 1/p^2, 1], {p, 2, n}]]; (* Michael Somos, Nov 08 2014 *)
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PROG
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(PARI) {a(n) = if(n<3, 0, 1 + n^2 * (n-6) / 24 * prod(p=2, n, if( isprime(p) && (n%p==0), 1 - 1/p^2, 1)))}; /* Michael Somos, May 19 2004 */
(PARI)
a(n) = {
if (n < 6, return(0));
my(f = factor(n), fsz = matsize(f)[1],
g = prod(k=1, fsz, f[k, 1]),
h = prod(k=1, fsz, sqr(f[k, 1]) - 1));
return(1 + (n-6)*sqr(n\g)*h\24);
};
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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