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199 9 1999 \large 1999^{1999}

Let N N denote the value of the number above, and we define f ( x ) f(x) as the sum of digits of integer x x . What is the value of f ( f ( f ( f ( N ) ) ) ) f(f(f(f(N)))) ?

3 5 2 4 1

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1 solution

Abdeslem Smahi
Aug 8, 2015

we know that N f ( N ) f ( f ( N ) ) f ( f ( f ( N ) ) ) ( m o d 9 ) N \equiv f(N) \equiv f(f(N)) \equiv f(f(f(N))) \pmod{9} .

From another side :

N = 199 9 1999 < 200 0 2000 = 2 2000 × 1 0 2000 = 102 4 200 × 1 0 6000 N=1999^{1999} < 2000^{2000} = 2^{2000} \times 10^{2000} =1024^{200} \times 10^{6000}

N < 1 0 800 × 1 0 6000 = 1 0 6800 \implies N < 10^{800} \times 10^{6000} = 10^{6800}

So N < 1 0 6800 f ( N ) < 6800 × 9 = 61200 f ( f ( N ) ) < 5 × 9 = 45 N < 10^{6800} \implies f(N) < 6800 \times 9=61200 \implies f(f(N)) < 5 \times 9=45 f ( f ( f ( N ) ) ) < 2 × 9 = 18 \implies f(f(f(N))) < 2 \times 9 = 18

we know that N f ( f ( f ( N ) ) ) 199 9 1999 1 1999 1 ( m o d 9 ) N \equiv f(f(f(N))) \equiv 1999^{1999} \equiv 1^{1999} \equiv 1 \pmod{9}

So f ( f ( f ( N ) ) ) 1 ( m o d 9 ) f(f(f(N))) \equiv 1 \pmod{9} and f ( f ( f ( N ) ) ) < 18 f(f(f(N))) < 18

f ( f ( f ( N ) ) ) = 10 \implies f(f(f(N)))=10

so f ( f ( f ( f ( N ) ) ) ) f(f(f(f(N)))) is 1 1

Moderator note:

Great! The key point here is to take advantage of the divisibility rules of 9.

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