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a(n) = ceiling(1000*log_10(n)).
4

%I #10 Dec 07 2019 02:59:33

%S 0,302,478,603,699,779,846,904,955,1000,1042,1080,1114,1147,1177,1205,

%T 1231,1256,1279,1302,1323,1343,1362,1381,1398,1415,1432,1448,1463,

%U 1478,1492,1506,1519,1532,1545

%N a(n) = ceiling(1000*log_10(n)).

%t Ceiling[1000*Log10[Range[40]]] (* _Harvey P. Dale_, Jul 19 2018 *)

%K nonn

%O 1,2

%A _N. J. A. Sloane_