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Question: Error from pdsolve using periodic BC.

Question:Error from pdsolve using periodic BC.

nm 8552 Maple 2019
pde pdsolve differential-equations
September 20 2019
0

Why Maple 2019.1 gives an error when no initial conditions are given for the following heat PDE with periodic BC?

I am using Physics 426 (current version). On windows 10.

When adding ic as some arbitrary function f(x), then the error goes away. But no ic needs to be given to solve this PDE. The answer can be left using arbitrary constants in this case.

I also found that this seems to happen when the BC are periodic. When using the normal Dirichlet B.C. and omitting the initial conditions, the error went away.

Am I doing something wrong or is this a bug?

> 

restart;
> 

pde:=diff(u(x,t),t)=diff(u(x,t),x$2); #try with NO IC
bc:=u(-Pi,t)=u(Pi,t),D[1](u)(-Pi,t)=D[1](u)(Pi,t);
pdsolve([pde,bc],u(x,t))

diff(u(x, t), t) = diff(diff(u(x, t), x), x)

u(-Pi, t) = u(Pi, t), (D[1](u))(-Pi, t) = (D[1](u))(Pi, t)

Error, (in pdsolve/BC/2nd_order/Series/TwoBC) invalid boolean expression: NULL

> 

restart;
> 

pde:=diff(u(x,t),t)=diff(u(x,t),x$2)-u(x,t); #now try with IC
bc:=u(-Pi,t)=u(Pi,t),D[1](u)(-Pi,t)=D[1](u)(Pi,t);
ic:=u(x,0)=f(x);
pdsolve([pde,bc,ic],u(x,t)); #solution is correct
> 

 

diff(u(x, t), t) = diff(diff(u(x, t), x), x)-u(x, t)

u(-Pi, t) = u(Pi, t), (D[1](u))(-Pi, t) = (D[1](u))(Pi, t)

u(x, 0) = f(x)

u(x, t) = exp(-t)*_C7[0]+Sum(exp(-t*(n^2+1))*(sin(n*x)*_C1[n]+cos(n*x)*_C7[n]), n = 1 .. infinity)

> 

restart;
> 

pde:=diff(u(x,t),t)=diff(u(x,t),x$2); #now try with NO IC, but not periodic BC
bc:=u(0,t)=1,u(Pi,t)=0;
pdsolve([pde,bc],u(x,t)); #solution is correct

diff(u(x, t), t) = diff(diff(u(x, t), x), x)

u(0, t) = 1, u(Pi, t) = 0

u(x, t) = ((Sum(sin(n*x)*exp(-n^2*t)*_C1(n), n = 1 .. infinity))*Pi+Pi-x)/Pi

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