function dc = fun(t,c,L)
%%%%%%Lc
dc=zeros(L,1);
dc1=zeros(L,1);
N=L/2;
% % NN=zeros(n,1);
% % for i=1:n
% % NN(i,1)=i-N-1;
% % end
%%%%%%%%%%%%L=4,N=2,。
% % % % % dc(1) = -12*pi*1i*c(3)*c(1)-64*1i*pi^3*c(1);
% % % % % dc(2) = 6*1i*pi*(c(3)*c(2)-c(1)*c(4))-8*1i*pi^3*c(2);
% % % % % dc(3) = 0;
% % % % % dc(4) =6*1i*pi*c(3)*c(4)+8*pi^3*1i*c(4);
for i=1:L
k=i-N-1;%%%%%c
dc(i)=dc(i)-8*1i*pi^3*k^3*c(i);
%%%%%%
for m0=1:L
m=m0-N-1;
n=k-m;
if (n>=(-N))(n<=(N-1))
n0=n+N+1;
dc1(i)=dc1(i)+n*c(m0)*c(n0);
end
end
dc(i)=dc(i)-6*1i*pi*dc1(i);
end