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 A074314 Deficient triangular numbers. 1
 1, 3, 10, 15, 21, 45, 55, 91, 105, 136, 153, 171, 190, 231, 253, 325, 351, 406, 435, 465, 561, 595, 703, 741, 861, 903, 946, 1035, 1081, 1225, 1275, 1378, 1431, 1485, 1653, 1711, 1891, 1953, 2145, 2211, 2278, 2415, 2485, 2701, 2775, 2926, 3003, 3081, 3321 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Intersection of A000217 and A005100. - Altug Alkan, Mar 22 2018 LINKS Muniru A Asiru, Table of n, a(n) for n = 1..2000 EXAMPLE 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. MAPLE with(numtheory): [select(n -> sigma(n) < 2*n, [seq(k*(k+1)/2, k=1..100)])]; # Muniru A Asiru, Mar 22 2018 PROG (GAP) Filtered(List([1..100], k->k*(k+1)/2), n->Sigma(n)<2*n); # Muniru A Asiru, Mar 22 2018 CROSSREFS Cf. A000217, A005100. Sequence in context: A328987 A108770 A112355 * A077759 A278291 A247420 Adjacent sequences:  A074311 A074312 A074313 * A074315 A074316 A074317 KEYWORD base,nonn AUTHOR Shyam Sunder Gupta, Sep 22 2002 STATUS approved

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Last modified September 22 17:39 EDT 2021. Contains 347607 sequences. (Running on oeis4.)