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%I #20 Dec 05 2012 03:17:53
%S 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,
%T 17,17,17,17,17,19,19,19,19,19,19,19,23,23,23,23,23,23,23,23,29,29,29,
%U 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
%N Denominators of terms of the sequence {c(n)} defined in A218121.
%C It is easy to see that every prime is in the sequence.
%p ispfree := proc(a,b)
%p local alow ;
%p alow := floor(a);
%p if nextprime(alow) < b then
%p false;
%p else
%p true;
%p end if;
%p end proc:
%p A218121c := proc(n)
%p option remember;
%p local k ;
%p if n = 1 then
%p return 1;
%p elif n = 2 then
%p return 5/2 ;
%p else
%p if ispfree(ithprime(n)/procname(n-1),ithprime(n+1)/procname(n-1)) then
%p return procname(n-1) ;
%p end if ;
%p for k from n by -1 do
%p if ispfree( ithprime(n)*ithprime(k)/ithprime(n+1),ithprime(k) )
%p and ithprime(n+1)/ithprime(k) > procname(n-1) then
%p return ithprime(n+1)/ithprime(k) ;
%p end if;
%p end do:
%p end if;
%p end proc:
%p A218123 := proc(n)
%p denom(A218121c(n)) ;
%p end proc: # _R. J. Mathar_, Dec 02 2012
%Y Cf. A218121, A217871, A217689, A217691, A217833, A217884.
%K nonn,frac
%O 1,2
%A _Vladimir Shevelev_, Oct 21 2012
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