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 A001766 Index of (the image of) the modular group Gamma(n) in PSL_2(Z). (Formerly M4098 N1700) 3
 1, 6, 12, 24, 60, 72, 168, 192, 324, 360, 660, 576, 1092, 1008, 1440, 1536, 2448, 1944, 3420, 2880, 4032, 3960, 6072, 4608, 7500, 6552, 8748, 8064, 12180, 8640, 14880, 12288, 15840, 14688, 20160, 15552, 25308, 20520, 26208, 23040, 34440, 24192, 39732, 31680 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Equivalently, the degree of the modular curve X(N) as a cover of the j-line. REFERENCES R. C. Gunning, Lectures on Modular Forms. Princeton Univ. Press, Princeton, NJ, 1962, p. 15. B. Schoeneberg, Elliptic Modular Functions, Springer-Verlag, NY, 1974, p. 76. 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). LINKS T. D. Noe, Table of n, a(n) for n = 1..1000 FORMULA a(n) = n * A000114(n). - Michael Somos, Jan 29 2004 MAPLE proc(n) local b, d: b := (n^3)/2: for d from 1 to n do if irem(n, d) = 0 and isprime(d) then b := b*(1-d^(-2)): fi: od: RETURN(b): end: MATHEMATICA Table[ (n^3)/If[ n>2, 2, 1 ] Times@@(1-1/Select[ Range[ n ], (Mod[ n, #1 ]==0&&PrimeQ[ #1 ])& ]^2), {n, 1, 45} ] (* Olivier Gérard, Aug 15 1997 *) CROSSREFS Equals A000056(n) for n = 2 and 1/2 * A000056(n) for n > 2 (since -I is contained in Gamma(2) but not in Gamma(n) for n > 2). Sequence in context: A091629 A089529 A300915 * A110959 A303398 A244743 Adjacent sequences:  A001763 A001764 A001765 * A001767 A001768 A001769 KEYWORD nonn,easy AUTHOR EXTENSIONS More terms from Olivier Gérard, Aug 15 1997 Definition corrected by Mira Bernstein, May 30 2006 STATUS approved

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Last modified January 16 23:44 EST 2019. Contains 319206 sequences. (Running on oeis4.)