# Greetings from The On-Line Encyclopedia of Integer Sequences! http://oeis.org/ Search: id:a273012 Showing 1-1 of 1 %I A273012 #27 Nov 11 2024 05:24:51 %S A273012 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 A273012 60,40,64,42,56,72,60,46,72,52,72,88,58,96,110,60,80,84,108,66,92,70, %U A273012 120,112,72,120,78,104,132,82,156,116,88,120,144,160,96,132,100 %N A273012 Totient of the n-th semiprime. %H A273012 Amiram Eldar, Table of n, a(n) for n = 1..10000 %F A273012 a(n) = phi(semiprime(n)) = A000010(A001358(n)), where phi is Euler's totient function. %F A273012 If A001358(n) = p^2, then a(n) = p*(p-1). %F A273012 If A001358(n) = p*q where p and q are distinct, then a(n) = (p-1)*(q-1). %F A273012 If A001358(n) = A006881(i), then a(n) = n+1-A228578(i). - _R. J. Mathar_, Nov 13 2016 %e A273012 a(3) = 6 because A000010(9) = 2*3 = 6. %p A273012 A273012 := proc(n) %p A273012 numtheory[phi](A001358(n)) ; %p A273012 end proc: %p A273012 seq(A273012(n),n=1..40) ; # _R. J. Mathar_, Nov 13 2016 %t A273012 EulerPhi@ Select[Range@ 202, PrimeOmega@ # == 2 &] (* _Michael De Vlieger_, May 13 2016 *) %o A273012 (PARI) lista(nn) = for(n=1, nn, if(bigomega(n) == 2, print1(eulerphi(n), ", "))); %Y A273012 Cf. A000010, A001358, A006093 (totients of primes), A006881, A228578. %K A273012 nonn,changed %O A273012 1,1 %A A273012 _Altug Alkan_, May 13 2016 # Content is available under The OEIS End-User License Agreement: http://oeis.org/LICENSE