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A128951 a(n) is equal to the number of positive integers m less than or equal to 10^n such that m is not divisible by the prime 5 and is not divisible by at least one of the primes 2, 3 and 7.

%I A128951 #15 Sep 08 2022 08:45:30
%S A128951 78,781,7809,78096,780952,7809524,78095238,780952381,7809523809,
%T A128951 78095238096,780952380952,7809523809524,78095238095238,
%U A128951 780952380952381,7809523809523809,78095238095238096,780952380952380952
%N A128951 a(n) is equal to the number of positive integers m less than or equal to 10^n such that m is not divisible by the prime 5 and is not divisible by at least one of the primes 2, 3 and 7.
%H A128951 Vincenzo Librandi, <a href="/A128951/b128951.txt">Table of n, a(n) for n = 2..1000</a>
%H A128951 Milan Janjic, <a href="http://www.pmfbl.org/janjic/">Enumerative Formulas for Some Functions on Finite Sets</a>
%F A128951 a(n) = 10^n - floor(10^n/5) - floor(10^n/42) + floor(10^n/210).
%e A128951 a(6) = 10^6 - floor(10^6/5) - floor(10^6/42) + floor(10^6/210) = 1000000 - floor(200000) - floor(23809.523...) + floor(4761.904...) = 1000000 - 200000 - 23809 + 4761 = 780952.
%p A128951 f := n->10^n-floor(10^n/5)-floor(10^n/42)+floor(10^n/210);
%t A128951 Table[With[{c=10^n},c-Floor[c/5]-Floor[c/42]+Floor[c/210]],{n,2,20}] (* _Harvey P. Dale_, Nov 02 2019 *)
%o A128951 (Magma) [10^n-Floor(10^n/5)-Floor(10^n/42)+Floor(10^n/210): n in [2..20]]; // _Vincenzo Librandi_, Oct 02 2011
%Y A128951 Cf. A092695.
%K A128951 nonn
%O A128951 2,1
%A A128951 _Milan Janjic_, Apr 28 2007
%E A128951 Example edited by _Jon E. Schoenfield_, Nov 18 2018