function L=ycdt(j0,jm,F)% y=0/ym 的邊界條件
global I c dt dx dy
ib=2:I-1;
L=-F(ib,jm,1)...
   +(c*dt-dy)/(c*dt+dy)*(F(ib,jm,3)+F(ib,j0,1))...
   +2*dy/(c*dt+dy)*(F(ib,j0,2)+F(ib,jm,2))...
   +(c*dt)^2*dy/(2*dx^2*(c*dt+dy))*(F(ib+1,j0,2)-2*F(ib,j0,2)+F(ib-1,j0,2)+F(ib+1,jm,2)-2*F(ib,jm,2)+F(ib-1,jm,2));