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A005381 Numbers n such that n and n-1 are composite.
(Formerly M4598)
26
9, 10, 15, 16, 21, 22, 25, 26, 27, 28, 33, 34, 35, 36, 39, 40, 45, 46, 49, 50, 51, 52, 55, 56, 57, 58, 63, 64, 65, 66, 69, 70, 75, 76, 77, 78, 81, 82, 85, 86, 87, 88, 91, 92, 93, 94, 95, 96, 99, 100, 105, 106, 111, 112, 115, 116, 117, 118, 119, 120, 121 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Position where the composites first outnumber the primes by n, among the first natural numbers. - Lekraj Beedassy, Jul 11 2006

REFERENCES

M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 844.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

J. Stauduhar, Table of n, a(n) for n = 1..10000

M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].

R. P. Boas & N. J. A. Sloane, Correspondence, 1974

B. W. J. Irwin, Recursive Modular Conjecture for pi(n).

FORMULA

Conjecture: pi(n)=Sum_{k=1..n} k mod a(m) mod a(m-1) ... mod a(1) mod 2, for all values 1<n<=a(m), where the mod are evaluated from left to right. Verified for first 10000 a(n). - Benedict W. J. Irwin, May 04 2016

As a check, take n=9, m=2, a(m)=10. Then we must take the numbers 1 through 9 and reduce them mod 10 then mod 9 then mod 2. The results are 1,0,1,0,1,0,1,0,0, whose sum is 4 = pi(9), as predicted. - N. J. A. Sloane, May 05 2016

For an attempt at a proof for the conjecture above, see the link. If it is true, then for n>2, isprime(n)=(n mod x) mod 2, where x is the largest a(n)<=n. - Benedict W. J. Irwin, May 06 2016

MAPLE

isA005381 := proc(n)

    not isprime(n) and not isprime(n-1) ;

end proc:

A005381 := proc(n)

    local a;

    option remember;

    if n = 1 then

        9;

    else

        for a from procname(n-1)+1 do

            if isA005381(a) then

                return a;

            end if;

        end do:

    end if;

end proc: # R. J. Mathar, Jul 14 2015

MATHEMATICA

Select[Range[2, 200], ! PrimeQ[# - 1] && ! PrimeQ[#] &]

PROG

(PARI) is(n)=!isprime(n)&&!isprime(n-1) \\ M. F. Hasler, Jan 07 2019

CROSSREFS

Equals A068780 + 1. Cf. A007921.

Cf. A093515 (complement, apart from 1 which is in neither sequence), A323162 (characteristic function).

Sequence in context: A227943 A114844 A194593 * A175090 A197113 A099616

Adjacent sequences:  A005378 A005379 A005380 * A005382 A005383 A005384

KEYWORD

nonn,easy,changed

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified January 18 06:34 EST 2019. Contains 319269 sequences. (Running on oeis4.)