A273012 - OEIS

A273012

Totient of the n-th semiprime.

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, 60, 40, 64, 42, 56, 72, 60, 46, 72, 52, 72, 88, 58, 96, 110, 60, 80, 84, 108, 66, 92, 70, 120, 112, 72, 120, 78, 104, 132, 82, 156, 116, 88, 120, 144, 160, 96, 132, 100

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

FORMULA

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

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

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

If A001358(n) = A006881(i), then a(n) = n+1-A228578(i). - R. J. Mathar, Nov 13 2016

EXAMPLE

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

MAPLE

A273012 := proc(n)

numtheory[phi](A001358(n)) ;

end proc:

seq(A273012(n), n=1..40) ; # R. J. Mathar, Nov 13 2016

MATHEMATICA

EulerPhi@ Select[Range@ 202, PrimeOmega@ # == 2 &] (* Michael De Vlieger, May 13 2016 *)

PROG

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

CROSSREFS

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

Sequence in context: A036500 A371824 A077080 * A222404 A081111 A092686

Adjacent sequences: A273009 A273010 A273011 * A273013 A273014 A273015

KEYWORD

nonn,changed

AUTHOR

Altug Alkan, May 13 2016

STATUS

approved