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Number of 2's in decimal expansion of triangular number n(n+1)/2.
9

%I #7 Dec 15 2017 17:36:32

%S 0,0,0,0,0,0,1,1,0,0,0,0,0,0,0,1,0,0,0,0,1,1,1,1,0,1,0,0,0,0,0,0,1,0,

%T 0,0,0,0,0,0,1,0,0,0,0,0,0,1,0,2,1,1,0,0,0,0,0,0,0,0,0,0,0,1,1,1,2,2,

%U 1,1,1,1,2,1,1,1,2,0,0,0,1,1,0,0,0,0,0,1,0,0,0,0,1,0,0,0,0,0,0,0,0,0,1,0,0

%N Number of 2's in decimal expansion of triangular number n(n+1)/2.

%e tri(49)=1225, so a(49)=2 and tri(651)=212226, so a(651)=4.

%t DigitCount[#,10,2]&/@Accumulate[Range[0,120]] (* _Harvey P. Dale_, Jan 02 2016 *)

%Y Cf. 0's A086071, 1's A086072, 3's A086074, 4's A086075, 5's A086076, 6's A086077, 7's A086078, 8's A086079, 9's A086080.

%K base,nonn

%O 0,50

%A _Jason Earls_, Jul 08 2003