%I #27 Nov 11 2024 05:24:51

%S 2,2,6,4,6,8,12,10,20,12,20,16,24,18,24,22,42,32,40,36,28,30,48,44,36,

%T 60,40,64,42,56,72,60,46,72,52,72,88,58,96,110,60,80,84,108,66,92,70,

%U 120,112,72,120,78,104,132,82,156,116,88,120,144,160,96,132,100

%N Totient of the n-th semiprime.

%H Amiram Eldar, <a href="/A273012/b273012.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = phi(semiprime(n)) = A000010(A001358(n)), where phi is Euler's totient function.

%F If A001358(n) = p^2, then a(n) = p*(p-1).

%F If A001358(n) = p*q where p and q are distinct, then a(n) = (p-1)*(q-1).

%F If A001358(n) = A006881(i), then a(n) = n+1-A228578(i). - _R. J. Mathar_, Nov 13 2016

%e a(3) = 6 because A000010(9) = 2*3 = 6.

%p A273012 := proc(n)

%p numtheory[phi](A001358(n)) ;

%p end proc:

%p seq(A273012(n),n=1..40) ; # _R. J. Mathar_, Nov 13 2016

%t EulerPhi@ Select[Range@ 202, PrimeOmega@ # == 2 &] (* _Michael De Vlieger_, May 13 2016 *)

%o (PARI) lista(nn) = for(n=1, nn, if(bigomega(n) == 2, print1(eulerphi(n), ", ")));

%Y Cf. A000010, A001358, A006093 (totients of primes), A006881, A228578.

%K nonn,changed

%O 1,1

%A _Altug Alkan_, May 13 2016