Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #15 Jul 31 2020 01:34:11
%S 1,9,25,54,93,24,192,72,464,606,40,9,302,9,88,69,464,9,1056,9,108,117,
%T 25,9,775,24,25,606,156,9,207,9,464,54,25,87,1166,9,25,54,479,9,255,9,
%U 93,621,25,9,775,72,40,54,93,9,1056,24,527,54,25,9,317,9,25
%N a(n) is the number of n-digit terms in A336668 (assuming 0 has 0 digit).
%C This sequence is bounded as the decimal representation of any term in A336668 is fully determined by at most 9 of its leading digits.
%H Rémy Sigrist, <a href="/A336669/b336669.txt">Table of n, a(n) for n = 0..10000</a>
%H Rémy Sigrist, <a href="/A336669/a336669.gp.txt">PARI program for A336669</a>
%F a(n) = 9 iff n is 11-rough (A008364).
%F a(k*n) >= a(n) for any n >= 0 and k > 0.
%F Apparently, when n > 0, a(n) = a(gcd(n, 2^3 * 3^2 * 5 * 7)).
%e For n = 2:
%e - let m be a two-digit term of A336668 (10 <= m <= 99),
%e - if m starts with an odd digit, say d = 1, 3, 5, 7 or 9, then m ends with d,
%e - if m starts with an even digit, say d = 2, 4, 6 or 8, then m ends with any even digit, say t = 0, 2, 4, 6 or 8,
%e - so a(2) = 5 + 4*5 = 25.
%o (PARI) See Links section.
%Y Cf. A008364, A336668.
%K nonn,base
%O 0,2
%A _Rémy Sigrist_, Jul 29 2020