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a(n) = phi(n) - d(n), where d(n) is the number of divisors function (A000005).
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%I #23 Nov 24 2021 19:34:14

%S 0,-1,0,-1,2,-2,4,0,3,0,8,-2,10,2,4,3,14,0,16,2,8,6,20,0,17,8,14,6,26,

%T 0,28,10,16,12,20,3,34,14,20,8,38,4,40,14,18,18,44,6,39,14,28,18,50,

%U 10,36,16,32,24,56,4,58,26,30,25,44,12,64,26,40,16,68,12,70,32,34,30,56,16,76,22,49,36,80,12,60,38,52,32,86,12,68,38

%N a(n) = phi(n) - d(n), where d(n) is the number of divisors function (A000005).

%C It is known that a(n) >= 1 for n >= 31.

%D D. S. Mitrinovic et al., Handbook of Number Theory, Kluwer, p. 11.

%H T. D. Noe, <a href="/A063070/b063070.txt">Table of n, a(n) for n=1..1000</a>

%F a(n) = A000010(n) - A000005(n). - _Wesley Ivan Hurt_, Nov 24 2021

%t Table[EulerPhi[n] - DivisorSigma[0, n], {n, 100}] (* _Wesley Ivan Hurt_, Nov 24 2021 *)

%o (PARI) j=[]; for(n=1,150,j=concat(j,eulerphi(n)-(numdiv(n)))); j

%o (PARI) { for (n=1, 1000, write("b063070.txt", n, " ", eulerphi(n) - numdiv(n)) ) } \\ _Harry J. Smith_, Aug 16 2009

%Y Cf. A000010, A000005. A020488 gives n such that a(n) = 0.

%K easy,sign

%O 1,5

%A _Jason Earls_, Aug 04 2001