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 A185981 #19 Sep 08 2022 08:45:55
%S A185981 1,256,281474976710656,
%T A185981 452312848583266388373324160190187140051835877600158453279131187530910662656
%N A185981 a(n) = 2^(2*(5^(n-1) - 1)).
%C A185981 The number of digits of a(n) is 1, 3, 15, 75, 376, 1881, 9407, 47036, 235180, 1175898, ....
%C A185981 -1/(4*a(n)) is the coefficient of x^0 of the minimal polynomial Psi(5^n,x) of cos(2*Pi/5^n). Hence 4*a(n)*Psi(5^n,x) is the integer polynomial with coefficient -1 of x^0. E.g., Psi(5,1)= x^2 + (1/2)*x -1/4, Psi(25,x)= x^10 + ... -1/1024. See A181875/A181876, A181877 and the W. Lang link under A181875.
%H A185981 <a href="/index/Di#divseq">Index to divisibility sequences</a>
%F A185981 a(n) = 2^(2*(5^(n-1) - 1)).
%t A185981 Table[2^(2*(5^(n-1)-1)), {n,1,10}] (* _G. C. Greubel_, Jul 24 2017 *)
%o A185981 (Magma) [(2^(2*(5^((n-1)))-1)/2): n in [1..5]]; // _Vincenzo Librandi_, Apr 19 2011
%o A185981 (PARI) a(n)=1<<(2*(5^(n-1)-1)) \\ _Charles R Greathouse IV_, Jan 13 2012
%Y A185981 Cf. A023365.
%K A185981 nonn,easy
%O A185981 1,2
%A A185981 _Wolfdieter Lang_, Feb 24 2011