A287355 - OEIS

%I #14 May 26 2017 21:24:58

%S 1,1,2,3,3,3,4,5,5,5,6,6,7,7,7,8,8,9,9,9,9,10,10,11,11,11,11,12,12,12,

%T 13,13,13,13,14,14,14,15,15,15,15,16,16,16,17,17,17,17,17,18

%N Order of largest subset of the positive rationals with neither the sum of numerators nor of denominators exceeding n.

%e For n = 1, the largest subset is { 1/1 }, so a(1) = 1; for n = 2, the same; for n = 3, largest subsets are { 1/1, 2/1 } and { 1/2, 1/1 }, so a(3) = 2; for n = 4, the largest subset is { 1/2, 1/1, 2/1 }, so a(4) = 3; ...; for n = 16, the largest subset is { 1/4, 1/3, 1/2, 2/3, 1/1, 2/1, 3/1, 4/1 } (or swap 2/3 for 3/2), so a(16)=8, etc.

%o (Haskell)

%o f = go 0 2

%o   where

%o     go a r n

%o       | n >= c             = go (a+t) (r+1) (n-c)

%o       | n >= r*div n r + m = a + 2*div n r + 1

%o       | n >= r*div n r + m' + 1 = a + 2*div n r + 1

%o       | otherwise          = a + 2*div n r

%o       where

%o         t  = totient r

%o         c  = div (r*t) 2

%o         m  = midnum r

%o         m' = midnum (r-1)

%o     midnum r = head [a|a<-[div (r+1) 2..], gcd a r==1]

%K nonn

%O 1,3

%A _Carl Edman_, May 23 2017