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a(n) = if n < 10 then n else a(floor(n/10) + n mod 10) + n mod 10.
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%I #13 Jan 23 2022 07:31:20

%S 0,1,2,3,4,5,6,7,8,9,1,3,5,7,9,11,13,15,17,10,2,4,6,8,10,12,14,16,9,

%T 12,3,5,7,9,11,13,15,8,11,14,4,6,8,10,12,14,7,10,13,16,5,7,9,11,13,6,

%U 9,12,15,18,6,8,10,12,5,8,11,14,17,20,7,9,11,4,7

%N a(n) = if n < 10 then n else a(floor(n/10) + n mod 10) + n mod 10.

%C a(A259046(n)) = n and a(m) != n for m < A259046(n).

%H Reinhard Zumkeller, <a href="/A259043/b259043.txt">Table of n, a(n) for n = 0..10000</a>

%o (Haskell)

%o a259043 x = if x < 10 then x else a259043 (x' + d) + d

%o where (x', d) = divMod x 10

%o (PARI) a(n) = if (n<10, n, my(u=n%10); a(n\10 + u) + u); \\ _Michel Marcus_, Jan 23 2022

%Y Cf. A076314, A007953, A008687, A259046.

%K nonn,base

%O 0,3

%A _Reinhard Zumkeller_, Jun 17 2015