(* Fibonacci numbers with memoization *)

fib::usage = "f[n] calculates the n'th Fibonacci number.";
fib[0] = fib[1] = 1;
fib[n_Integer?Positive]:= fib[n] = fib[n-1] + fib[n-2];

In[4]:= fib[42]
Out[4]= 433494437
