Example 5.11
| > | restart; |
| > | with(linalg):with(plots): |
| > | ge:=diff(u(x,t),t)=-diff(u(x,t),x); |
| (1) |
| > | bc1:=u(x,t)-1; |
| (2) |
| > | bc2:=diff(u(x,t),t)=-diff(u(x,t),x); |
| (3) |
| > | IC:=u(x,0)=0; |
| (4) |
| > | N:=200; |
| (5) |
| > | L:=1; |
| (6) |
| > | dydxf:=1/2*(-u[2](t)-3*u[0](t)+4*u[1](t))/h: |
| > | #dydxb:=1/2*(u[N-1](t)+3*u[N+1](t)-4*u[N](t))/h: |
| > | dydxb:=(u[N+1](t)-u[N](t))/h: |
| > | dydx:=1/h*(u[m](t)-u[m-1](t)): |
| > | #dydx:=1/2/h*(u[m+1](t)-u[m-1](t)): |
| > | d2ydx2:=1/h^2*(u[m-1](t)-2*u[m](t)+u[m+1](t)): |
| > | bc1:=subs(diff(u(x,t),x)=dydxf,u(x,t)=u[0](t),x=0,bc1): |
| > | bc2:=subs(diff(u(x,t),x)=dydxb,u(x,t)=u[N+1](t),x=1,bc2): |
| > | eq[0]:=bc1; |
| (7) |
| > | eq[N+1]:=bc2; |
| (8) |
| > | for i from 1 to N do eq[i]:=diff(u[i](t),t)= subs(diff(u(x,t),x$2) = subs(m=i,d2ydx2),diff(u(x,t),x) = subs(m=i,dydx),u(x,t)=u[i](t),x=i*h,rhs(ge));od: |
| > | u[0](t):=(solve(eq[0],u[0](t))); |
| (9) |
| > | for i from 1 to N+1 do eq[i]:=eval(eq[i]);od: |
| > | eqs:=[seq(rhs(eq[j]),j=1..N+1)]: |
| > | Y:=[seq(u[i](t),i=1..N+1)]: |
| > | A:=genmatrix(eqs,Y,'b1'): |
| > | b:=matrix(N+1,1):for i to N+1 do b[i,1]:=-eval(b1[i]);od:evalm(b): |
| > | h:=eval(L/(N+1)): |
| > | A:=map(eval,A): |
| > | if N > 4 then A:=map(evalf,A);end: |
| > | Nrow:=rowdim(A); |
| (10) |
| > | #exponential(A,t); |
| > | #seq([seq(exp(-Nrow*t)*(Nrow*t)^(i-j),j=1..i)],i=1..Nrow); |
| > | mat:=Matrix(Nrow,[seq([seq(exp(-Nrow*t)*(Nrow*t)^(i-j)/factorial(i-j),j=1..i)],i=1..Nrow)],shape=triangular[lower]): |
| > | Y0:=matrix(N+1,1):for i from 1 to N+1 do Y0[i,1]:=evalf(subs(x=i*h,rhs(IC)));od:evalm(Y0): |
| > | Y:=evalm(mat&*(Y0+inverse(A)&*b)-inverse(A)&*b): |
| > | #b2:=subs(t=tau,evalm(b)): |
| > | #mat2:=subs(t=t-tau,evalm(mat)): |
| > | #mat3:=evalm(mat2&*b2): |
| > | #mat4:=map(int,mat3,tau=0..t): |
| > | #Y:=evalm(mat&*Y0+mat4): |
| > | for i from 1 to N+1 do u[i](t):=evalf((Y[i,1]));od: |
| > | for i from 0 to N+1 do u[i](t):=eval(u[i](t));od: |
| > | for i from 0 to N+1 do u[i](t):=subs(I=0,u[i](t));od: |
| > | setcolors(["Red", "Blue", "LimeGreen", "Goldenrod", "maroon", "DarkTurquoise", "coral", "aquamarine", "magenta", "khaki", "sienna", "orange", "yellow", "gray"]): |
| > | pp:=plot([seq(subs(delta=1,u[i](t)),i=0..N+1,20)],t=0..1,thickness=3); |
| (11) |
| > | pt:=textplot([seq([0.05+i/20*0.09,evalf(subs(delta=1,t=0.05+i/20*0.09,u[i](t))),typeset(u[i]),align={below,right}],i=0..N+1,20)]); |
| (12) |
| > | display([pp,pt],title="Figure Exp. 5.34.",axes=boxed,labels=[t,"u"]); |
![]() |
| > | for i from 0 to N+1 do p[i]:=plot(subs(delta=0.1,u[i](t)),t=0..0.4,thickness=3);od: |
| > | tf:=1; |
| (13) |
| > | M:=30; |
| (14) |
| > | T1:=[seq(tf*i/M,i=0..M)]: |
| > | PP:=matrix(N+2,M+1); |
| (15) |
| > | for i from 1 to N+2 do PP[i,1]:=evalf(subs(x=(i-1)*h,rhs(IC)));od: |
| > | for i from 1 to N+2 do for j from 2 to M+1 do PP[i,j]:=evalf(subs(t=T1[j],subs(delta=1,u[i-1](t))));od;od: |
| > | plotdata := [seq([ seq([(i-1)*h,T1[j],PP[i,j]], i=1..N+2)], j=1..M+1)]: |
| > | surfdata(plotdata,title="Figure Exp. 5.35.",axes=boxed, labels=[x,t,u],orientation=[-45,45]); |
![]() |
| > |