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%I #13 Jun 12 2023 18:10:20
%S 1,3,10,15,21,45,55,91,105,136,153,171,190,231,253,325,351,406,435,
%T 465,561,595,703,741,861,903,946,1035,1081,1225,1275,1378,1431,1485,
%U 1653,1711,1891,1953,2145,2211,2278,2415,2485,2701,2775,2926,3003,3081,3321
%N Deficient triangular numbers.
%C Intersection of A000217 and A005100. - _Altug Alkan_, Mar 22 2018
%H Muniru A Asiru, <a href="/A074314/b074314.txt">Table of n, a(n) for n = 1..2000</a>
%e a(5)=21 because sum of aliquot divisors of 21( which is a triangular number) is 1+3+7=11 which is less than 21, hence it is deficient. 21 is 5th deficient triangular number.
%p with(numtheory): [select(n -> sigma(n) < 2*n, [seq(k*(k+1)/2, k=1..100)])]; # _Muniru A Asiru_, Mar 22 2018
%t Select[Accumulate[Range[100]],DivisorSigma[1,#]<2#&] (* _Harvey P. Dale_, Jun 12 2023 *)
%o (GAP) Filtered(List([1..100],k->k*(k+1)/2),n->Sigma(n)<2*n); # _Muniru A Asiru_, Mar 22 2018
%Y Cf. A000217, A005100.
%K base,nonn
%O 1,2
%A _Shyam Sunder Gupta_, Sep 22 2002
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