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A029936 Number of cusps of Gamma_1(n)\P_1(Q). 3
1, 2, 2, 3, 4, 4, 6, 6, 8, 8, 10, 10, 12, 12, 16, 14, 16, 16, 18, 20, 24, 20, 22, 24, 28, 24, 30, 30, 28, 32, 30, 32, 40, 32, 48, 40, 36, 36, 48, 48, 40, 48, 42, 50, 64, 44, 46, 56, 60, 56, 64, 60, 52, 60, 80, 72, 72, 56 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

REFERENCES

F. Hirzebruch et al., Manifolds and Modular Forms, Vieweg, 2nd ed. 1994, p. 158.

LINKS

G. C. Greubel, Table of n, a(n) for n = 1..10000

FORMULA

Except for n=1, 2, 4, this is A029935(n)/2.

a(n) = (1/2)*Sum_{d divides n} phi(d)*phi(n/d), with a(1)=1, a(2)=2, a(3)=2, a(4)=3, and phi(n) = A000010(n). - G. C. Greubel, Dec 13 2017

MATHEMATICA

a[1] = 1; a[2] = 2; a[4] = 3; a[n_] := DivisorSum[n, EulerPhi[#]* EulerPhi[n/#]&]/2; Array[a, 60] (* Jean-Fran├žois Alcover, Oct 03 2016 *)

PROG

(PARI) for(n=1, 30, print1(if(n==1, 1, if(n==2, 2, if(n==3, 2, if(n==4, 3, sumdiv(n, d, eulerphi(d)*eulerphi(n/d))/2)))), ", ")) \\ G. C. Greubel, Dec 13 2017

CROSSREFS

Sequence in context: A138369 A173332 A138374 * A114093 A239950 A077768

Adjacent sequences:  A029933 A029934 A029935 * A029937 A029938 A029939

KEYWORD

nonn,nice

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified December 15 17:03 EST 2019. Contains 330000 sequences. (Running on oeis4.)