login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A071975 Denominator of rational number i/j such that Sagher map sends i/j to n. 4
1, 2, 3, 1, 5, 6, 7, 4, 1, 10, 11, 3, 13, 14, 15, 1, 17, 2, 19, 5, 21, 22, 23, 12, 1, 26, 9, 7, 29, 30, 31, 8, 33, 34, 35, 1, 37, 38, 39, 20, 41, 42, 43, 11, 5, 46, 47, 3, 1, 2, 51, 13, 53, 18, 55, 28, 57, 58, 59, 15, 61, 62, 7, 1, 65, 66, 67, 17, 69, 70, 71, 4, 73, 74, 3, 19, 77 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
The Sagher map sends Product p_i^e_i / Product q_i^f_i (p_i and q_i being distinct primes) to Product p_i^(2e_i) * Product q_i^(2f_i-1). This is multiplicative.
LINKS
Yoram Sagher, Counting the rationals, Amer. Math. Monthly, 96 (1989), p. 823. Math. Rev. 90i:04001.
FORMULA
If n = Product p_i^e_i, then a(n) = Product p_i^f(e_i), where f(n) = (n+1)/2 if n is odd and f(n) = 0 if n is even. - Reiner Martin, Jul 08 2002
From Reinhard Zumkeller, Jul 10 2011: (Start)
a(n^2) = 1, A071974(n^2) = n, cf. A000290.
a(2*(2*n-1)^2) = 2, A071974(2*(2*n-1)^2) = 2*n+1, cf. A077591.
a(2*(2*n-1)^2) = 2, A071974(2*(2*n-1)^2) = 2*n+1, cf. A077591. (End)
Sum_{k=1..n} a(k) ~ c * n^2, where c = (Pi^4*zeta(3)/180) * Product_{p prime} (1 - 1/p^2 - 1/p^3 + 1/p^4 - 1/p^5 + 1/p^6) = 0.3394877587... . - Amiram Eldar, Oct 30 2022
EXAMPLE
The Sagher map sends the following fractions to 1, 2, 3, 4, ...: 1/1, 1/2, 1/3, 2/1, 1/5, 1/6, 1/7, 1/4, 3/1, ...
MATHEMATICA
f[{p_, a_}] := If[OddQ[a], p^((a+1)/2), 1]; a[n_] := Times@@(f/@FactorInteger[n])
PROG
(PARI) a(n)=local(v=factor(n)~); prod(k=1, length(v), if(v[2, k]%2, v[1, k]^-(-v[2, k]\2), 1))
(Haskell)
a071975 n = product $ zipWith (^) (a027748_row n) $
map (\e -> (e `mod` 2) * (e + 1) `div` 2) $ a124010_row n
-- Reinhard Zumkeller, Jun 15 2012
CROSSREFS
Sequence in context: A327938 A065883 A214392 * A350389 A368886 A182659
KEYWORD
nonn,frac,easy,nice,mult
AUTHOR
N. J. A. Sloane, Jun 19 2002
EXTENSIONS
More terms from Reiner Martin, Jul 08 2002
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 29 02:23 EDT 2024. Contains 371264 sequences. (Running on oeis4.)