OFFSET
1,1
COMMENTS
Subsequence of A195238. - Harvey P. Dale, Sep 13 2011
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
FORMULA
A143201(a(n)) = 6. - Harvey P. Dale, Sep 13 2011
Sum_{n>=1} 1/a(n) = 7/24. - Amiram Eldar, Oct 25 2024
EXAMPLE
a(1) = 14 = 2 * 7 = A033847(1).
a(2) = 28 = 2^2 * 7 = A033847(2).
a(3) = 30 = 2 * 3 * 5 = A143207(1).
a(4) = 56 = 2^3 * 7 = A033847(3).
a(5) = 60 = 2^2 * 3 * 5 = A143207(2).
a(6) = 90 = 2 * 3^2 * 5 = A143207(3).
a(7) = 98 = 2 * 7^2 = A033847(4).
a(8) = 112 = 2^4 * 7 = A033847(5).
a(9) = 120 = 2^3 * 3 * 5 = A143207(4).
a(10) = 150 = 2 * 3 * 5^2 = A143207(5).
a(11) = 180 = 2^2 * 3^2 * 5 = A143207(6).
a(12) = 196 = 2^2 * 7^2 = A033847(6).
MATHEMATICA
q[n_] := Module[{p1 = {2, 3, 5}, p2 = {2, 7}, e1, e2}, e1 = IntegerExponent[n, p1]; e2 = IntegerExponent[n, p2]; (Times @@ e1 > 0 && Times @@ (p1^e1) == n) || (Times @@ e2 > 0 && Times @@ (p2^e2) == n)]; Select[Range[3000], q] (* Amiram Eldar, Oct 25 2024 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Reinhard Zumkeller, Aug 12 2008
EXTENSIONS
Corrected by Harvey P. Dale, Aug 21 2011
Revised version with improved definition; thanks to Harvey P. Dale, who noticed that the original definition was not sufficient.
STATUS
approved