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%I #11 Nov 28 2017 14:10:09
%S 4,6,26,80,246,810,2704,9252,32066,112720,400024,1432860,5170604,
%T 18784170,68635478,252088496,930138522,3446167860,12815663844,
%U 47820447028,178987624514,671825133648,2528212128776,9536895064400
%N Number of elements in the mutation class of a quiver of type D_n.
%C Table 1, p. 15 of Buan et al.
%C Except for a(4) = 6 the same as A003239. - _Joerg Arndt_, Aug 04 2014
%H Bakke Buan, Hermund André Torkildsen, <a href="http://arxiv.org/abs/0812.2240">The number of elements in the mutation class of a quiver of type $D_n$</a>, arXiv:0812.2240 [math.RT], (14-April-2009)
%F a(n) = 6 if n = 4; otherwise a(n) = SUM[d|n] (phi(n/d))C(2d,d)/(2n) where phi is the Euler function, when n>4.
%F For n>4 a(n) = SUM[d|n] A000010(n/d)*A000984(d)/(2*n)
%p A159557 := proc(n) if n = 3 then 4; elif n = 4 then 6; else add( numtheory[phi](n/d)*binomial(2*d,d),d=numtheory[divisors](n))/2/n ; fi; end: seq(A159557(n),n=3..40) ; # _R. J. Mathar_, Apr 16 2009
%t a[4] = 6; a[n_] := Sum[EulerPhi[n/d]*Binomial[2d, d]/(2n), {d, Divisors[n]} ];
%t Table[a[n], {n, 3, 26}] (* _Jean-François Alcover_, Nov 28 2017 *)
%Y Cf. A000010, A000984.
%K nonn
%O 3,1
%A _Jonathan Vos Post_, Apr 15 2009
%E More terms from _R. J. Mathar_, Apr 16 2009
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