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A225505 a(n) = triangular(a(n-1)+a(n-2)) if n > 1, else a(n) = n.

%I A225505 #9 Dec 26 2024 19:53:13
%S A225505 0,1,1,3,10,91,5151,13741903,94490753712985,
%T A225505 4464252567106907867941273716,
%U A225505 9964775491460730298984873904585383048580645394630925051
%N A225505 a(n) = triangular(a(n-1)+a(n-2)) if n > 1, else a(n) = n.
%o A225505 (Python)
%o A225505 prpr, prev = 0, 1
%o A225505 for i in range(1, 17):
%o A225505   print(prpr, end=', ')
%o A225505   n = prpr+prev
%o A225505   cur = n*(n+1)//2
%o A225505   prpr, prev = prev, cur
%Y A225505 Cf. A000217, A072638.
%K A225505 nonn
%O A225505 0,4
%A A225505 _Alex Ratushnyak_, May 09 2013