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Efficient exponentiation
Problem 122
Published on 02 June 2006 at 06:00 pm [Server Time]
The most naive way of computing n15 requires fourteen multiplications:
n × n × ... × n = n15
But using a "binary" method you can compute it in six multiplications:
n × n = n2
n2 × n2 = n4
n4 × n4 = n8
n8 × n4 = n12
n12 × n2 = n14
n14 × n = n15
However it is yet possible to compute it in only five multiplications:
n × n = n2
n2 × n = n3
n3 × n3 = n6
n6 × n6 = n12
n12 × n3 = n15
We shall define m(k) to be the minimum number of multiplications to compute nk; for example m(15) = 5.
For 1 ≤ k ≤ 200, find ∑ m(k).
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