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%I #15 Dec 12 2023 13:46:49
%S 1,2,3,5,4,8,11,6,15,12,9,7,20,25,16,30,36,10,18,47,27,38,59,13,58,73,
%T 43,81,19,26,96,14,45,109,121,72,54,44,70,17,98,137,60,156,29,113,155,
%U 92,22,145,173,63,112,46,39,136,204,24,219,174,21,237,253,80
%N a(n) is the least positive integer not already appearing such that no two distinct, unordered pairs of distinct points ((n, a(n)), (m, a(m))) and ((k, a(k)), (j, a(j))) have the same midpoint.
%H Giovanni Resta, <a href="/A285492/b285492.txt">Table of n, a(n) for n = 1..3000</a> (first 650 terms from Peter Kagey)
%e a(4) != 4 or else midpoint((4,4), (1,1)) = midpoint((2,2), (3,3)), thus
%e a(4) = 5.
%e a(6) != 6 or else midpoint((6,6), (3,3)) = midpoint((4,5), (5,4));
%e a(6) != 7 or else midpoint((6,7), (1,1)) = midpoint((3,3), (4,5)); thus
%e a(6) = 8.
%Y Cf. A285490.
%K nonn,look
%O 1,2
%A _Peter Kagey_, Apr 19 2017
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