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%I #27 Aug 10 2019 05:35:01
%S 1,2,2,3,2,5,2,5,4,7,2,9,2,9,8,9,2,13,2,13,10,13,2,17,6,15,10,17,2,23,
%T 2,17,14,19,12,25,2,21,16,25,2,31,2,25,22,25,2,33,8,31,20,29,2,37,16,
%U 33,22,31,2,45,2,33,28,33,18,47,2,37,26,47,2
%N a(n) = n - phi(n) + 1.
%C This is the cototient(A051953) + 1. If n = p*q for different primes p and q, a(n) = p + q. - _Wesley Ivan Hurt_, Aug 27 2013
%C If n is the product of twin primes, (a(n) +- 2)/2 gives the two primes. - _Wesley Ivan Hurt_, Sep 06 2013
%H Antti Karttunen, <a href="/A062830/b062830.txt">Table of n, a(n) for n = 1..65537</a>
%F a(n) = A051953(n) + 1 = n - A000010(n) + 1. a(A006881(n)) = A008472(A006881(n)). - _Wesley Ivan Hurt_, Aug 27 2013
%F a(n) = 2*A067392(n)/n for n > 1. - _Robert G. Wilson v_, Jul 16 2019
%e a(10) = 7, since 10 - phi(10) + 1 = 10 - 4 + 1 = 7. Also, since 10 is a squarefree semiprime, 7 represents the sum of the distinct prime factors of 10.
%p with(numtheory); seq(k - phi(k) + 1, k = 1..70); # _Wesley Ivan Hurt_, Aug 27 2013
%t Table[n - EulerPhi[n] + 1, {n, 100}] (* _Wesley Ivan Hurt_, Aug 27 2013 *)
%o (PARI) j=[]; for(n=1,200,j=concat(j,eulerphi(n)-n-1)); j
%Y Cf. A000010, A006881, A008472, A051953.
%K easy,nonn
%O 1,2
%A _Jason Earls_, Jul 20 2001
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