MathDB
Digital sum squared

Source: Kürschák 1992, problem 2

July 20, 2014
modular arithmeticnumber theory unsolvednumber theory

Problem Statement

For any positive integer kk define f1(k)f_1(k) as the square of the digital sum of kk in the decimal system, and fn(k)=f1(fn−1(k))f_{n}(k)=f_1(f_{n-1}(k)) ∀n>1\forall n>1. Compute f1992(21991)f_{1992}(2^{1991}).
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