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a(n) = t(n)_t(n) where t() = triangular numbers A000217.
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%I #7 Apr 10 2021 20:20:52

%S 1,3,6,10,20,43,64,114,185,280,402,554,820,11030,14640,18910,24177,

%T 30439,37810,88410,107416,129286,154290,270000,317530,371359,431306,

%U 659350,758210,867695,988534,1394984,1576972,1775485,2383290,2665338,3459466,3846532

%N a(n) = t(n)_t(n) where t() = triangular numbers A000217.

%C See A122618 for notation.

%H Alois P. Heinz, <a href="/A122628/b122628.txt">Table of n, a(n) for n = 1..10000</a>

%p a:= n-> (b-> (l-> add(l[i]*b^(i-1),

%p i=1..nops(l)))(convert(b, base, 10)))(n*(n+1)/2):

%p seq(a(n), n=1..35); # _Alois P. Heinz_, Apr 10 2021

%K nonn,base

%O 1,2

%A _N. J. A. Sloane_, Sep 21 2006