%I A001766 M4098 N1700
%S A001766 1,6,12,24,60,72,168,192,324,360,660,576,1092,1008,1440,1536,2448,
%T A001766 1944,3420,2880,4032,3960,6072,4608,7500,6552,8748,8064,12180,8640,
%U A001766 14880,12288,15840,14688,20160,15552,25308,20520,26208,23040,34440
%N A001766 Index of (the image of) the modular group Gamma(n) in PSL_2(Z).
%C A001766 Equivalently, the degree of the modular curve X(N) as a cover of the 
               j-line.
%C A001766 a(n)=n*A000114(n). - Michael Somos Jan 29 2004
%D A001766 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A001766 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 
               (includes this sequence).
%D A001766 R. C. Gunning, Lectures on Modular Forms. Princeton Univ. Press, Princeton, 
               NJ, 1962, p. 15.
%D A001766 B. Schoeneberg, Elliptic Modular Functions, Springer-Verlag, NY, 1974, 
               p. 76.
%H A001766 <a href="Sindx_Gre.html#groups_modular">Index entries for sequences related 
               to modular groups</a>
%p A001766 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:
%t A001766 Table[ (n^3)/If[ n>2, 2, 1 ] Times@@(1-1/Select[ Range[ n ], (Mod[ n, 
               #1 ]==0&&PrimeQ[ #1 ])& ]^2), {n, 1, 45} ]
%Y A001766 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).
%Y A001766 Sequence in context: A082505 A091629 A089529 this_sequence A110959 A065106 
               A030775
%Y A001766 Adjacent sequences: A001763 A001764 A001765 this_sequence A001767 A001768 
               A001769
%K A001766 nonn,easy
%O A001766 1,2
%A A001766 N. J. A. Sloane (njas(AT)research.att.com).
%E A001766 More terms and Mathematica program Aug 15 1997 from Olivier Gerard.
%E A001766 Definition corrected by Mira Bernstein, May 30 2006