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a(n) = smallest k such that 5k has a digit sum = n.
2

%I #17 Oct 26 2020 07:56:52

%S 2,4,6,8,1,3,5,7,9,11,13,15,17,19,39,59,79,99,119,139,159,179,199,399,

%T 599,799,999,1199,1399,1599,1799,1999,3999,5999,7999,9999,11999,13999,

%U 15999,17999,19999,39999,59999,79999,99999,119999,139999,159999,179999

%N a(n) = smallest k such that 5k has a digit sum = n.

%H <a href="/index/Rec#order_10">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,0,0,0,0,0,0,10,-10).

%F a(n) = A069534(n)/5.

%F a(n) = a(n-1) + 10*a(n-9) - 10*a(n-10) for n > 14. - _Georg Fischer_, Oct 26, 2020

%t LinearRecurrence[{1, 0, 0, 0, 0, 0, 0, 0, 10, -10},{2, 4, 6, 8, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19}, 40] (* _Georg Fischer_, Oct 26 2020 *)

%Y Cf. A069532, A069534.

%K base,nonn,easy

%O 1,1

%A _Amarnath Murthy_, Nov 07 2002

%E More terms from _Ray Chandler_, Jul 28 2003