Let denote the value of the number above, and we define as the sum of digits of integer . What is the value of ?
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we know that N ≡ f ( N ) ≡ f ( f ( N ) ) ≡ f ( f ( f ( N ) ) ) ( m o d 9 ) .
From another side :
N = 1 9 9 9 1 9 9 9 < 2 0 0 0 2 0 0 0 = 2 2 0 0 0 × 1 0 2 0 0 0 = 1 0 2 4 2 0 0 × 1 0 6 0 0 0
⟹ N < 1 0 8 0 0 × 1 0 6 0 0 0 = 1 0 6 8 0 0
So N < 1 0 6 8 0 0 ⟹ f ( N ) < 6 8 0 0 × 9 = 6 1 2 0 0 ⟹ f ( f ( N ) ) < 5 × 9 = 4 5 ⟹ f ( f ( f ( N ) ) ) < 2 × 9 = 1 8
we know that N ≡ f ( f ( f ( N ) ) ) ≡ 1 9 9 9 1 9 9 9 ≡ 1 1 9 9 9 ≡ 1 ( m o d 9 )
So f ( f ( f ( N ) ) ) ≡ 1 ( m o d 9 ) and f ( f ( f ( N ) ) ) < 1 8
⟹ f ( f ( f ( N ) ) ) = 1 0
so f ( f ( f ( f ( N ) ) ) ) is 1