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A008328 Number of divisors of p-1, p prime. 12
1, 2, 3, 4, 4, 6, 5, 6, 4, 6, 8, 9, 8, 8, 4, 6, 4, 12, 8, 8, 12, 8, 4, 8, 12, 9, 8, 4, 12, 10, 12, 8, 8, 8, 6, 12, 12, 10, 4, 6, 4, 18, 8, 14, 9, 12, 16, 8, 4, 12, 8, 8, 20, 8, 9, 4, 6, 16, 12, 16, 8, 6, 12, 8, 16, 6, 16, 20, 4, 12, 12, 4, 8, 12, 16, 4, 6, 18, 15, 16, 8, 24, 8 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also the number of irreducible factors of Phi(p,x)-1, for cyclotomic polynomial Phi(p,x) and prime p. The formula is Phi(p,x)-1 = x Prod {n>1, n|p-1} Phi(n,x). - T. D. Noe, Oct 17 2003

LINKS

T. D. Noe, Table of n, a(n) for n=1..10000

Eric Weisstein's World of Mathematics, Cyclotomic Polynomial

FORMULA

a(n) = A000005(A006093(n)) = A066800(prime(n)). - R. J. Mathar, Oct 01 2017

MAPLE

for i from 1 to 500 do if isprime(i) then print(tau(i-1)); fi; od;

A008328 := proc(n)

    numtheory[tau](ithprime(n)-1) ;

end proc: # R. J. Mathar, Oct 30 2015

MATHEMATICA

DivisorSigma[0, #-1]&/@Prime[Range[90]] (* Harvey P. Dale, Dec 08 2011 *)

CROSSREFS

Sequence in context: A008329 A064558 A178031 * A298933 A091860 A301764

Adjacent sequences:  A008325 A008326 A008327 * A008329 A008330 A008331

KEYWORD

nonn

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified August 3 05:55 EDT 2020. Contains 336197 sequences. (Running on oeis4.)