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102

answers:

1

Assume an arbitrary r

T(n) <= cn + T(n/r) + T (3n/4)

show T(n) <= Dcn for some constant D

by reworking the induction proof, use the expression to argue that:

T(n) <= Dcn does not hold for r=3.

+1  A: 

Have a look at the Akra-Bazzi theorem. This is a generalization of the master theorem that does not require subproblems of equal size.

Daniel Brückner
but we havent taught that by Prof. yet
Nick
Yep. That's it. The induction method and a few examples here: http://www.mpi-inf.mpg.de/~mehlhorn/DatAlg2008/NewMasterTheorem.pdf
belisarius