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1, 2, 2, 4, 4, 4, 6, 8, 6, 8, 10, 8, 12, 12, 8, 16, 16, 12, 18, 16, 12, 20, 22, 16, 20, 24, 18, 24, 28, 16, 30, 32, 20, 32, 24, 24, 36, 36, 24, 32, 40, 24, 42, 40, 24, 44, 46, 32, 42, 40, 32, 48, 52, 36, 40, 48, 36, 56, 58, 32, 60, 60, 36, 64, 48, 40, 66, 64, 44, 48, 70, 48, 72
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| a(n) is also the number of non-congruent solutions to x^2 - y^2 = 1 mod n. - Yuval Dekel (dekelyuval(AT)hotmail.com), Sep 21 2003
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FORMULA
| a(n) = Sum_{d divides n and d is odd} n/d*mu(d).
Multiplicative with a(2^e) = 2^e and a(p^e) = p^e-p^(e-1), p>2.
Dirichlet g.f.: zeta(s-1)/zeta(s)*2^s/(2^s-1). - Ralf Stephan, Jun 17 2007
a(n) = A000010(2*n).
a(n) = phi(n)*(1+((n+1) mod 2)). [From Gary Detlefs, Jul 13 2011]
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PROG
| (PARI) a(n)=if(n<1, 0, eulerphi(2*n))
(Sage) [euler_phi(2*n) for n in xrange(1, 74)] # [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jun 06 2009]
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CROSSREFS
| Cf. A000010, A008683, A062803, A060968, A000034.
Sequence in context: A206224 A035114 A202103 * A108514 A120456 A115383
Adjacent sequences: A062567 A062568 A062569 * A062571 A062572 A062573
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KEYWORD
| mult,nonn
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AUTHOR
| Jason Earls (zevi_35711(AT)yahoo.com), Jul 03 2001
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EXTENSIONS
| Corrected by Vladeta Jovovic (vladeta(AT)eunet.rs), Dec 04 2002
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