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A218123 Denominators of terms of the sequence {c(n)} defined in A218121. 2
1, 2, 2, 2, 2, 3, 3, 3, 5, 5, 5, 7, 7, 7, 7, 7, 7, 11, 11, 11, 11, 13, 13, 13, 13, 13, 13, 17, 17, 17, 17, 17, 17, 19, 19, 19, 19, 19, 19, 19, 23, 23, 23, 23, 23, 23, 23, 23, 29, 29, 29, 29, 31, 31, 31, 31, 31, 31, 31, 31, 31, 37, 37, 37, 37, 37, 37, 41, 41, 41, 41, 43, 43, 43, 43, 43, 43, 43, 47, 47 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

It is easy to see that every prime is in the sequence.

LINKS

Table of n, a(n) for n=1..80.

MAPLE

ispfree := proc(a, b)

    local alow ;

    alow := floor(a);

    if nextprime(alow) < b then

        false;

    else

        true;

    end if;

end proc:

A218121c := proc(n)

    option remember;

    local k ;

    if n = 1 then

        return 1;

    elif n = 2 then

        return 5/2 ;

    else

        if ispfree(ithprime(n)/procname(n-1), ithprime(n+1)/procname(n-1)) then

            return procname(n-1) ;

        end if ;

        for k from n by -1 do

            if ispfree( ithprime(n)*ithprime(k)/ithprime(n+1), ithprime(k) )

                and ithprime(n+1)/ithprime(k) > procname(n-1) then

                return ithprime(n+1)/ithprime(k) ;

            end if;

        end do:

    end if;

end proc:

A218123 := proc(n)

    denom(A218121c(n)) ;

end proc: # R. J. Mathar, Dec 02 2012

CROSSREFS

Cf. A218121, A217871, A217689, A217691, A217833, A217884.

Sequence in context: A029084 A032229 A024366 * A288157 A333701 A140682

Adjacent sequences:  A218120 A218121 A218122 * A218124 A218125 A218126

KEYWORD

nonn,frac

AUTHOR

Vladimir Shevelev, Oct 21 2012

STATUS

approved

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Last modified May 18 22:37 EDT 2022. Contains 353826 sequences. (Running on oeis4.)