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!)
A094457 a(n) is the largest number k < n in a neighborhood of n defined by prime factorization, specifically, A127185(n,k) <= 2. 6
1, 2, 2, 3, 4, 5, 4, 6, 6, 7, 8, 11, 10, 10, 8, 13, 12, 17, 12, 15, 14, 19, 16, 15, 22, 18, 20, 23, 20, 29, 16, 22, 26, 25, 24, 31, 34, 33, 24, 37, 30, 41, 28, 30, 38, 43, 32, 35, 30, 39, 44, 47, 36, 35, 40, 51, 46, 53, 40, 59, 58, 45, 32, 55, 44, 61, 52, 57, 50, 67, 48, 71, 62 (list; graph; refs; listen; history; text; internal format)
OFFSET
2,2
COMMENTS
Old name: Greatest number strictly less than n with same factorization pattern.
Here we call a number "comparable" if it is in the specified neighborhood of n. For n > 1, A064802(n) is the next larger comparable number. If n is not a power of 2, the ratio of a(n) to n is the ratio of successive primes.
LINKS
EXAMPLE
a(18)=12 because 18=2*3*3 and next smaller comparable factorization is 2*2*3=12 (A127185(18,12) = 2).
MATHEMATICA
f[n_] := Block[{g, h}, g[x_] := Flatten[Table[#1, {#2}] & @@@ FactorInteger@ x]; h[x_] := If[x == 2, 1, NextPrime[x, -1]]; If[n == 1, 1, Max[Times @@ MapAt[h, g[n], #] & /@ Range[Length@ g[n]]]]]; Table[f@ i, {i, 2, 74}] (* Michael De Vlieger, Jan 31 2015 *)
CROSSREFS
Sequence in context: A281487 A224401 A252462 * A275495 A022820 A292259
KEYWORD
easy,nonn
AUTHOR
Marc LeBrun, May 06 2004
EXTENSIONS
Edited, with new name, by Peter Munn, Oct 10 2023
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 2 00:03 EST 2024. Contains 370447 sequences. (Running on oeis4.)