A074314 Deficient triangular numbers.

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

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

Mathematica

Select[Accumulate[Range[100]], DivisorSigma[1, #]<2#&] (* Harvey P. Dale, Jun 12 2023 *)

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