login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A143207 Numbers with distinct prime factors 2, 3, and 5. 22

%I

%S 30,60,90,120,150,180,240,270,300,360,450,480,540,600,720,750,810,900,

%T 960,1080,1200,1350,1440,1500,1620,1800,1920,2160,2250,2400,2430,2700,

%U 2880,3000,3240,3600,3750,3840,4050,4320,4500,4800,4860

%N Numbers with distinct prime factors 2, 3, and 5.

%C A001221(a(n)) = 3; A020639(a(n)) = 2; A006530(a(n)) = 5; A143201(a(n)) = 6.

%C Subsequence of A143204 and of A051037.

%C Successive k such that EulerPhi[x]/x=4/15. - _Artur Jasinski_, Nov 07 2008

%C Numbers of the form 2^i * 3^j * 5^k with i, j, k > 0; a(n) = 30*A051037(n); A007947(a(n)) = A010869(n). - _Reinhard Zumkeller_, Sep 13 2011

%H T. D. Noe, <a href="/A143207/b143207.txt">Table of n, a(n) for n = 1..1000</a>

%t a = {}; Do[If[EulerPhi[x]/x == 4/15, AppendTo[a, x]], {x, 1, 11664}]; a (* _Artur Jasinski_, Nov 07 2008 *)

%o (Haskell)

%o import Data.Set (singleton, deleteFindMin, insert)

%o a143207 n = a143207_list !! (n-1)

%o a143207_list = f (singleton (2*3*5)) where

%o f s = m : f (insert (2*m) $ insert (3*m) $ insert (5*m) s') where

%o (m,s') = deleteFindMin s

%o -- _Reinhard Zumkeller_, Sep 13 2011

%o (PARI) is(n)=if(n%30, return(0)); my(g=30); while(g>1, n/=30; g=gcd(n,30)); n==1 \\ _Charles R Greathouse IV_, Sep 14 2015

%o (PARI) list(lim)=my(v=List(),s,t); for(i=1,logint(lim\6,5), t=5^i; for(j=1,logint(lim\t\2,3), s=t*3^j; while((s<<=1)<=lim, listput(v,s)))); Set(v) \\ _Charles R Greathouse IV_, Sep 14 2015

%o (MAGMA) [n: n in [1..5000] | PrimeDivisors(n) eq [2,3,5]]; // _Bruno Berselli_, Sep 14 2015

%Y Cf. A069819.

%K nonn,easy

%O 1,1

%A _Reinhard Zumkeller_, Aug 12 2008

%E New name from _Charles R Greathouse IV_, Sep 14 2015

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified November 13 04:20 EST 2019. Contains 329085 sequences. (Running on oeis4.)