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 A112742 #36 Feb 16 2025 08:32:59
%S A112742 0,0,4,24,80,200,420,784,1344,2160,3300,4840,6864,9464,12740,16800,
%T A112742 21760,27744,34884,43320,53200,64680,77924,93104,110400,130000,152100,
%U A112742 176904,204624,235480,269700,307520,349184,394944,445060,499800,559440
%N A112742 a(n) = n^2*(n^2 - 1)/3.
%C A112742 Second derivative of the n-th Chebyshev polynomial (of the first kind) evaluated at x=1.
%C A112742 The second derivative at x=-1 is just (-1)^n * a(n).
%C A112742 The difference between two consecutive terms generates the sequence a(n+1) - a(n) = A002492(n).
%C A112742 Consider the partitions of 2n into two parts (p,q) where p <= q. Then a(n) is the total volume of the family of rectangular prisms with dimensions p, |q-p| and |q-p|. - _Wesley Ivan Hurt_, Apr 15 2018
%H A112742 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ChebyshevPolynomialoftheFirstKind.html">Chebyshev polynomials of the first kind</a>
%H A112742 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (5,-10,10,-5,1).
%F A112742 a(n) = (n-1)*n^2*(n+1)/3 = 4*A002415(n).
%F A112742 a(n) = 2*( A000914(n-1) + C(n+1,4) ). - _David Scambler_, Nov 27 2006
%F A112742 From _Colin Barker_, Jan 26 2012: (Start)
%F A112742 a(n) = 5*a(n-1) - 10*a(n-2) + 10*a(n-3) - 5*a(n-4) + a(n-5).
%F A112742 G.f.: 4*x^2*(1+x)/(1-x)^5. (End)
%F A112742 E.g.f.: exp(x)*x^2*(6 + 6*x + x^2)/3. - _Stefano Spezia_, Dec 11 2021
%F A112742 a(n) = A053126(n+2) - A006324(n-1). - _Yasser Arath Chavez Reyes_, Feb 22 2024
%e A112742 a(4)=80 because
%e A112742 C_4(x) = 1 - 8x^2 + 8x^4,
%e A112742 C'_4(x) = -16x + 32x^3,
%e A112742 C''_4(x) = -16 + 96x^2,
%e A112742 C''_4(1) = -16 + 96 = 80.
%t A112742 Table[D[ChebyshevT[n, x], {x, 2}], {n, 0, 100}] /. x -> 1
%o A112742 (PARI) a(n)=n^2*(n^2-1)/3 \\ _Charles R Greathouse IV_, Oct 07 2015
%Y A112742 Cf. A000914, A002415, A002492.
%K A112742 nonn,easy
%O A112742 0,3
%A A112742 Matthew T. Cornick (maruth(AT)gmail.com), Sep 16 2005