This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A381488 #20 Mar 03 2025 12:42:52
%S A381488 1,5,22,35,51,92,117,145,247,287,376,425,477,590,651,715,782,925,1001,
%T A381488 1162,1247,1335,1426,1617,1717,2035,2147,2501,2625,2882,3015,3151,
%U A381488 3577,3725,4187,4347,4845,5017,5551,5735,6112,6305,6501,6902,7107,7315,7526,7957
%N A381488 Pentagonal numbers that are deficient.
%H A381488 Vincenzo Librandi, <a href="/A381488/b381488.txt">Table of n, a(n) for n = 1..10046</a>
%e A381488 22 = 2*11 is the 4th pentagonal number and is a deficient number, since it is larger than the sum of its proper divisors (14).
%e A381488 117 = 3^2*13 is the 9th pentagonal number and is a deficient number, since it is larger than the sum of its proper divisors (65).
%e A381488 1001 = 7*11*13 is the 26th pentagonal number and is a deficient number, since it is larger than the sum of its proper divisors (343).
%t A381488 Select[Table[n*(3*n-1)/2, {n, 1, 75}], DivisorSigma[-1, #] < 2 &] (* _Amiram Eldar_, Feb 25 2025 *)
%o A381488 (PARI) select(x->sigma(x)<2*x, vector(100, k, k*(3*k-1)/2)) \\ _Michel Marcus_, Feb 25 2025
%o A381488 (Magma) filtered := [n*(3*n-1) div 2 : n in [1..80] | &+ [1/d : d in Divisors(n*(3*n-1) div 2)] lt 2]; filtered; // _Vincenzo Librandi_, Mar 03 2025
%Y A381488 Intersection of A005100 and A000326.
%Y A381488 Cf. A379264.
%K A381488 nonn
%O A381488 1,2
%A A381488 _Massimo Kofler_, Feb 25 2025