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 A129768 Number of odd nonprime numbers less than the n-th prime. 1
 1, 1, 1, 1, 2, 2, 3, 3, 4, 6, 6, 8, 9, 9, 10, 12, 14, 14, 16, 17, 17, 19, 20, 22, 25, 26, 26, 27, 27, 28, 34, 35, 37, 37, 41, 41, 43, 45, 46, 48, 50, 50, 54, 54, 55, 55, 60, 65, 66, 66, 67, 69, 69, 73, 75, 77, 79, 79, 81, 82, 82, 86, 92, 93, 93, 94, 100, 102, 106, 106, 107, 109 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS Also, a(n)=sum_{k=n-th even number..n-th odd prime together with 1}-k*(-1)^k. - Juri-Stepan Gerasimov, Jul 30 2009 LINKS Table of n, a(n) for n=1..72. FORMULA a(n)=A008507(n-1)+1. - R. J. Mathar, Jul 06 2009 a(n)=sum_{k=A005843(n)..A006005(n)}-k*(-1)^k. - Juri-Stepan Gerasimov, Jul 30 2009 MAPLE A129768 := proc(n) local res, p, i ; res := 0 ; p := ithprime(n) ; for i from 1 to p-1 by 2 do if not isprime(i) then res := res+1 ; fi ; od ; RETURN(res) ; end: for n from 1 to 80 do printf("%d, ", A129768(n)) ; od ; # R. J. Mathar, May 17 2007 MATHEMATICA f[n_] := Ceiling[ Prime@n / 2] - n + 1; Array[f, 72] (* Robert G. Wilson v *) nn=401; With[{np=Select[Range[1, nn, 2], !PrimeQ[#]&]}, Table[Count[np, _?(#

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Last modified March 1 22:23 EST 2024. Contains 370443 sequences. (Running on oeis4.)