Heat equation (not stationary)

[volcano63]

One more point - default variable values; instead of this monstrosity

def lambd(T,coeff1,coeff2):
     '''O si prends les valeurs exp, 1 si prends les valeurs entrées pour coeff1 et coeff2'''
     if coeff1==0 and coeff2==0:
           coeff1=lambda1
           coeff2=lambda2

that shorthand

def lambd(T, coeff1=lambda1, coeff2=lambda2):

And lambd(L[k][i],0,0) turns into lambd(L[k][i])

PS lambda being a reserved word in Python, using names very similar to it may be not such a good idea