Author 
Comment/Response 
jf

04/20/13 8:47pm
a) Is f already defined somewhere in your code?
Clear[f]
before running these inputs would eliminate that possibility.
b) DSolve needs both independent variables in the third argument.
c) DiracDelta is not a nice function. It would not surprise me if DSolve cannot do much with it.
d) That these just echo says DSolve did not know how to solve them.
In[1]:= eqns = {Derivative[0, 1][f][x, t] == a*Derivative[2, 0][f][x, t],
Derivative[1, 0][f][0, t] == 0, Derivative[1, 0][f][L, t] == 0,
f[x, 0] == DiracDelta[x]}
In[2]:= DSolve[eqns, f[x, t], {x, t}]
Out[2]=
DSolve[{f^(0,1)[x, t] == a f^(2,0)[x, t], f^(1,0)[0, t] == 0, f^(1,0)[L, t] == 0, f[x, 0] == DiracDelta[x]},
f[x, t], {x, t}]
In[3]:= DSolve[eqns, f, {x, t}]
Out[3]=
DSolve[{f^(0,1)[x, t] == a f^(2,0)[x, t], f^(1,0)[0, t] == 0, f^(1,0)[L, t] == 0, f[x, 0] == DiracDelta[x]},
f, {x, t}]
In[4]:= $Version
Out[4]= 9.0 for Microsoft Windows (64bit) (January 25, 2013)
URL: , 
