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 A163109 #23 May 09 2025 22:03:41
%S A163109 1,1,1,2,1,2,1,2,2,2,1,2,1,2,2,4,1,2,1,2,2,2,1,4,2,2,2,2,1,4,1,2,2,2,
%T A163109 2,6,1,2,2,4,1,4,1,2,2,2,1,4,2,2,2,2,1,4,2,4,2,2,1,4,1,2,2,6,2,4,1,2,
%U A163109 2,4,1,4,1,2,2,2,2,4,1,4,4,2,1,4,2,2,2,4,1,4,2,2,2,2,2,4,1,2,2,6,1,4,1,4,4
%N A163109 a(n) = phi(tau(n)).
%H A163109 Antti Karttunen, <a href="/A163109/b163109.txt">Table of n, a(n) for n = 1..10000</a>
%H A163109 <a href="/index/Eu#epf">Index entries for sequences computed from exponents in factorization of n</a>
%F A163109 a(n) = A000010(A000005(n)). - _Charles R Greathouse IV_, Aug 11 2009
%F A163109 a(1) = 1, a(p) = 1 for p = primes (A000040), a(p*q) = 2 for p*q = product of two distinct primes (A006881), a(p*q*...*z) = 2^(k-1) for p*q*...*z = product of k (k > 2) distinct primes p, q, ..., z (A120944), a(p^(q-1)) = q - 1 for p, q = primes (A000040).
%e A163109 a(16) = a(2^(5-1)) = 5-1 = 4.
%t A163109 Table[EulerPhi[DivisorSigma[0, n]], {n, 1, 80}] (* _Carl Najafi_, Aug 15 2011 *)
%o A163109 (PARI) a(n) = eulerphi(numdiv(n)); \\ _Michel Marcus_, Aug 22 2015
%Y A163109 Cf. A000005, A000010, A062821, A163377, A163378, A163379.
%K A163109 nonn,easy
%O A163109 1,4
%A A163109 _Jaroslav Krizek_, Jul 20 2009
%E A163109 More terms from _Carl Najafi_, Aug 15 2011
%E A163109 Further extended by _Antti Karttunen_, Jul 23 2017