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Original Message (ID '179008') By jf:
You are making assumptions about k that need to be explicit. In[1]:= f[t_, k_, a_] := a*t^k In[2]:= f[0, k, a] k Out[2]= 0 a In[3]:= Simplify[%, k > 0] Out[3]= 0 What would happen if k were negative? In[4]:= 0^(-3) -3 Power::infy: Infinite expression 0 encountered. Out[4]= ComplexInfinity If you really, really want f[0,..,..] to be zero regardless of the other arguments, you can make a second, more specific, definition. In[8]:= f[0, k_, a_] := 0 In[9]:= f[0, -2, 137] Out[9]= 0 Since this definition of f is more specific than the earlier definition, it will be tried first.