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(A259542(n) + A259542(n+1)) / 2.
6

%I #4 Jun 30 2015 17:14:24

%S 3,6,10,15,10,6,10,15,21,28,36,45,36,28,36,45,55,36,21,28,36,45,36,28,

%T 36,45,55,66,78,91,78,66,78,91,105,78,55,66,78,91,78,66,78,91,105,120,

%U 136,153,120,78,91,105,91,78,66,78,105,120,136,153,171,153

%N (A259542(n) + A259542(n+1)) / 2.

%C All terms are triangular numbers by definition of A259542.

%H Reinhard Zumkeller, <a href="/A259604/b259604.txt">Table of n, a(n) for n = 1..10000</a>

%o (Haskell)

%o a259604 n = a259604_list !! (n-1)

%o a259604_list = zipWith ((flip div 2 .) . (+))

%o a259542_list $ tail a259542_list

%Y Cf. A259542, A000217, A086517, A259602, A259603, A259605.

%K nonn

%O 1,1

%A _Reinhard Zumkeller_, Jun 30 2015