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 A100007 #16 Jan 28 2023 12:14:25
%S A100007 1,2,2,2,2,2,2,4,2,2,4,2,2,2,2,2,4,4,2,4,2,2,4,2,2,4,2,4,4,2,2,4,4,2,
%T A100007 4,2,2,4,4,2,2,2,4,4,2,4,4,4,2,4,2,2,8,2,2,4,2,4,4,4,2,4,2,2,4,2,4,4,
%U A100007 2,2,4,4,4,4,2,2,4,4,2,4,4,2,8,2,2,4,2,4,4,2,2,4,4,4,4,2,2,8,2,2,4,4,4,4,4
%N A100007 Number of unitary divisors of 2n-1 (d such that d divides 2n-1, GCD(d,(2n-1)/d)=1). Bisection of A034444.
%F A100007 From _Ilya Gutkovskiy_, Apr 28 2017: (Start)
%F A100007 a(n) = [x^(2*n-1)] Sum_{k>=1} mu(k)^2*x^k/(1 - x^k).
%F A100007 a(n) = 2^omega(2*n-1). (End)
%F A100007 From _Amiram Eldar_, Jan 28 2023: (Start)
%F A100007 a(n) = A034444(2*n-1) = A068068(2*n-1).
%F A100007 Sum_{k=1..n} a(k) ~ 4*n*((log(n) + 2*gamma - 1 + 7*log(2)/3 - 2*zeta'(2)/zeta(2)) / Pi^2, where gamma is Euler's constant (A001620). (End)
%e A100007 a(13)=2 because among the three divisors of 25 only 1 and 25 are unitary.
%p A100007 with(numtheory): for n from 1 to 120 do printf(`%d,`,2^nops(ifactors(2*n-1)[2])) od: # _Emeric Deutsch_, Dec 24 2004
%t A100007 a[n_] := 2^PrimeNu[2*n-1]; Array[a, 100] (* _Amiram Eldar_, Jan 28 2023 *)
%o A100007 (PARI) a(n) = 2^omega(2*n-1); \\ _Amiram Eldar_, Jan 28 2023
%Y A100007 Cf. A001620, A034444, A068068, A100008.
%K A100007 nonn,easy
%O A100007 1,2
%A A100007 _N. J. A. Sloane_, Nov 20 2004
%E A100007 More terms from _Emeric Deutsch_, Dec 24 2004