%I M0411 N0157
%S 1,1,1,2,3,1,5,4,3,3,9,2,11,5,3,8,15,3,17,6,5,9,21,4,15,11,9,10,27,3,
%T 29,16,9,15,15,6,35,17,11,12,39,5,41,18,9,21,45,8,35,15,15,22,51,9,27,
%U 20,17,27,57,6,59,29,15,32,33,9,65,30,21,15,69,12,71,35,15,34,45,11,77,24,27
%N Number of pairs x,y such that yx=2, (x,n)=1, (y,n)=1 and 1 <= x <= n.
%D Golubev, V. A.; Nombres de Mersenne et caracteres du nombre 2. Mathesis 67 1958 257262.
%D N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H T. D. Noe, <a href="/A002472/b002472.txt">Table of n, a(n) for n = 1..1000</a>
%F Multiplicative with a(p^e) = p^(e1) if p = 2; (p2)*p^(e1) if p > 2.  _David W. Wilson_, Aug 01, 2001.
%t a[n_] := If[ Head[ r=Reduce[ GCD[x, n] == 1 && GCD[x+2, n] == 1 && 1 <= x <= n, x, Integers]] === Or, Length[r], 1]; Table[a[n], {n, 1, 81}] (* _JeanFrançois Alcover_, Nov 22 2011 *)
%o (PARI) a(n)=my(k=valuation(n,2),f=factor(n>>k));prod(i=1,#f[,1],(f[i,1]2)*f[i,1]^(f[i,2]1))<<max(0,k1) \\ _Charles R Greathouse IV_, Nov 22 2011
%o (Haskell)
%o a002472 n = length [x  x < [1..n], gcd n x == 1, gcd n (x + 2) == 1]
%o  _Reinhard Zumkeller_, Mar 23 2012
%K nonn,nice,easy,mult
%O 1,4
%A _N. J. A. Sloane_.
%E More terms from _David W. Wilson_
