A165704 -id:A165704 - cp's OEIS frontend

A165705 Range and record values of A165704.

1, 3, 5, 7, 9, 11, 15, 17, 19, 23, 29, 33, 35, 41, 47, 55, 57, 59, 71, 73, 85, 93, 99, 109, 115, 123, 147, 149, 169, 175, 185, 187, 211, 223, 235, 255, 295, 297, 299, 307, 337, 361, 373, 383, 443, 457, 477, 517, 529, 589, 591, 599, 601, 641, 683, 743, 757, 787

Offset: 1

Reinhard Zumkeller, Sep 26 2009

nonn

a(1) = A165704(0); for n>1: a(n) = A165704(A051037(n)) and a(m) < A165704(m) for m < A051037(n).

Cf. A051037, A165704.

A051037 5-smooth numbers, i.e., numbers whose prime divisors are all <= 5.

Original entry on oeis.org

1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24, 25, 27, 30, 32, 36, 40, 45, 48, 50, 54, 60, 64, 72, 75, 80, 81, 90, 96, 100, 108, 120, 125, 128, 135, 144, 150, 160, 162, 180, 192, 200, 216, 225, 240, 243, 250, 256, 270, 288, 300, 320, 324, 360, 375, 384, 400, 405

Offset: 1

Eric W. Weisstein

Sometimes called the Hamming sequence, since Hamming asked for an efficient algorithm to generate the list, in ascending order, of all numbers of the form 2^i*3^j*5^k for i,j,k >= 0. The problem was popularized by Edsger Dijkstra.
Numbers k such that 8*k = EulerPhi(30*k). - Artur Jasinski, Nov 05 2008
Where record values greater than 1 occur in A165704: A165705(n) = A165704(a(n)). - Reinhard Zumkeller, Sep 26 2009
Also called "harmonic whole numbers", see Howard and Longair, 1982, Table I, page 121. - Hugo Pfoertner, Jul 16 2020
Also called ugly numbers, although it is not clear why. - Gus Wiseman, May 21 2021
Some woody bamboo species have extraordinarily long and stable flowering intervals that belong to this sequence. The model by Veller, Nowak & Davis justifies this observation from the evolutionary point of view. - Andrey Zabolotskiy, Jun 27 2021
Also those integers k for which, for every prime p > 5, p^(4*k) - 1 == 0 (mod 240*k). - Federico Provvedi, May 23 2022
As noted in the comments to A085152, Størmer's theorem implies that the only pairs of consecutive integers that appear as consecutive terms of this sequence are (1,2), (2,3), (3,4), (4,5), (5,6), (8,9), (9,10), (15,16), (24,25), and (80,81). These all represent significant musical intervals. - Hal M. Switkay, Dec 05 2022

			From _Gus Wiseman_, May 21 2021: (Start)
The sequence of terms together with their prime indices begins:
      1: {}            25: {3,3}
      2: {1}           27: {2,2,2}
      3: {2}           30: {1,2,3}
      4: {1,1}         32: {1,1,1,1,1}
      5: {3}           36: {1,1,2,2}
      6: {1,2}         40: {1,1,1,3}
      8: {1,1,1}       45: {2,2,3}
      9: {2,2}         48: {1,1,1,1,2}
     10: {1,3}         50: {1,3,3}
     12: {1,1,2}       54: {1,2,2,2}
     15: {2,3}         60: {1,1,2,3}
     16: {1,1,1,1}     64: {1,1,1,1,1,1}
     18: {1,2,2}       72: {1,1,1,2,2}
     20: {1,1,3}       75: {2,3,3}
     24: {1,1,1,2}     80: {1,1,1,1,3}
(End)

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 (first 1000 terms from T. D. Noe)
Benoit Cloitre, Plot of abs(f(n)-s(n)) vs its mean values (blue) and vs loglog(n) (red).
M. J. Dominus, Infinite Lists in Perl.
Deborah Howard and Malcolm Longair, Harmonic Proportion and Palladio's "Quattro Libri", Journal of the Society of Architectural Historians (1982) 41 (2): 116-143.
Vaclav Kotesovec, Plot of a(n) / (exp((6*log(2)*log(3)*log(5)*n)^(1/3))/sqrt(30)) for n = 1..1200000
Rosetta Code, A collection of computer codes to compute 5-smooth numbers.
Raphael Schumacher, The Formula for the Distribution of the 3-Smooth Numbers, 5-Smooth, 7-Smooth and all other Smooth Numbers, arXiv:1608.06928 [math.NT], 2016.
Sci.math, Ugly numbers.
Carl Veller, Martin A. Nowak and Charles C. Davis, Extended flowering intervals of bamboos evolved by discrete multiplication, Ecology Letters, 18 (2015), 653-659.
Eric Weisstein's World of Mathematics, Smooth Number.
Wikipedia, Regular number.
Wikipedia, Talk:Regular number. Includes a discussion of the name.
Wikipedia, Størmer's theorem.

Subsequences: A003592, A003593, A051916 , A257997.
For p-smooth numbers with other values of p, see A003586, A002473, A051038, A080197, A080681, A080682, A080683.
Cf. A006530, A112757, A112758, A112759, A112763, A112764, A291719.
The partitions with these Heinz numbers are counted by A001399.
The conjugate opposite is A033942, counted by A004250.
The opposite is A059485, counted by A004250.
The non-3-smooth case is A080193, counted by A069905.
The conjugate is A037144, counted by A001399.
The complement is A279622, counted by A035300.
Requiring the sum of prime indices to be even gives A344297.
Cf. A000244, A002182, A002183, A035301, A056239, A112798, A261144, A344293.

import Data.Set (singleton, deleteFindMin, insert)
a051037 n = a051037_list !! (n-1)
a051037_list = f $ singleton 1 where
   f s = y : f (insert (5 * y) $ insert (3 * y) $ insert (2 * y) s')
               where (y, s') = deleteFindMin s
-- Reinhard Zumkeller, May 16 2015

[n: n in [1..500] | PrimeDivisors(n) subset [2,3,5]]; // Bruno Berselli, Sep 24 2012

A051037 := proc(n)
    option remember;
    local a;
    if n = 1 then
        1;
    else
        for a from procname(n-1)+1 do
            numtheory[factorset](a) minus {2, 3,5 } ;
            if % = {} then
                return a;
            end if;
        end do:
    end if;
end proc:
seq(A051037(n),n=1..100) ; # R. J. Mathar, Nov 05 2017

mx = 405; Sort@ Flatten@ Table[ 2^a*3^b*5^c, {a, 0, Log[2, mx]}, {b, 0, Log[3, mx/2^a]}, {c, 0, Log[5, mx/(2^a*3^b)]}] (* Or *)
Select[ Range@ 405, Last@ Map[First, FactorInteger@ #] < 7 &] (* Robert G. Wilson v *)
With[{nn=10},Select[Union[Times@@@Flatten[Table[Tuples[{2,3,5},n],{n,0,nn}],1]],#<=2^nn&]] (* Harvey P. Dale, Feb 28 2022 *)

test(n)= {m=n; forprime(p=2,5, while(m%p==0,m=m/p)); return(m==1)}
for(n=1,500,if(test(n),print1(n",")))

a(n)=local(m); if(n<1,0,n=a(n-1); until(if(m=n, forprime(p=2,5, while(m%p==0,m/=p)); m==1),n++); n)

list(lim)=my(v=List(),s,t); for(i=0,logint(lim\=1,5), t=5^i; for(j=0,logint(lim\t,3), s=t*3^j; while(s<=lim, listput(v,s); s<<=1))); Set(v) \\ Charles R Greathouse IV, Sep 21 2011; updated Sep 19 2016

smooth(P:vec,lim)={ my(v=List([1]),nxt=vector(#P,i,1),indx,t);
while(1, t=vecmin(vector(#P,i,v[nxt[i]]*P[i]),&indx);
if(t>lim,break); if(t>v[#v],listput(v,t)); nxt[indx]++);
Vec(v)
};
smooth([2,3,5], 1e4) \\ Charles R Greathouse IV, Dec 03 2013

is_A051037(n)=n<7||vecmax(factor(n,6)[, 1])<7 \\ M. F. Hasler, Jan 16 2015

def isok(n):
  while n & 1 == 0: n >>= 1
  while n % 3 == 0: n //= 3
  while n % 5 == 0: n //= 5
  return n == 1 #  Darío Clavijo, Dec 30 2022

from sympy import integer_log
def A051037(n):
    def bisection(f,kmin=0,kmax=1):
        while f(kmax) > kmax: kmax <<= 1
        while kmax-kmin > 1:
            kmid = kmax+kmin>>1
            if f(kmid) <= kmid:
                kmax = kmid
            else:
                kmin = kmid
        return kmax
    def f(x):
        c = n+x
        for i in range(integer_log(x,5)[0]+1):
            for j in range(integer_log(y:=x//5**i,3)[0]+1):
                c -= (y//3**j).bit_length()
        return c
    return bisection(f,n,n) # Chai Wah Wu, Sep 16 2024

# faster for initial segment of sequence
import heapq
from itertools import islice
def A051037gen(): # generator of terms
    v, oldv, h, psmooth_primes, = 1, 0, [1], [2, 3, 5]
    while True:
        v = heapq.heappop(h)
        if v != oldv:
            yield v
            oldv = v
            for p in psmooth_primes:
                    heapq.heappush(h, v*p)
print(list(islice(A051037gen(), 65))) # Michael S. Branicky, Sep 17 2024

Let s(n) = Card(k | a(k)Benoit Cloitre, Dec 30 2001
The characteristic function of this sequence is given by:
  Sum_{n>=1} x^a(n) = Sum_{n>=1} -Möbius(30*n)*x^n/(1-x^n). - Paul D. Hanna, Sep 18 2011
a(n) = A143207(n) / 30. - Reinhard Zumkeller, Sep 13 2011
A204455(15*a(n)) = 15, and only for these numbers. - Wolfdieter Lang, Feb 04 2012
A006530(a(n)) <= 5. - Reinhard Zumkeller, May 16 2015
Sum_{n>=1} 1/a(n) = Product_{primes p <= 5} p/(p-1) = (2*3*5)/(1*2*4) = 15/4. - Amiram Eldar, Sep 22 2020

A083662 a(n) = a(floor(n/2)) + a(floor(n/4)), n > 0; a(0)=1.

Original entry on oeis.org

1, 2, 3, 3, 5, 5, 5, 5, 8, 8, 8, 8, 8, 8, 8, 8, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 34, 34, 34, 34, 34, 34, 34, 34, 34, 34, 34

Offset: 0

Benoit Cloitre, Oct 05 2003

nonn

A000045(n+2) = a(A131577(n))and A000045(m+2) < a(m) for m < A131577(n). - Reinhard Zumkeller, Sep 26 2009

R. Zumkeller, Table of n, a(n) for n = 0..10000

Cf. A088468.

A007731, A165704, A165706. - Reinhard Zumkeller, Sep 26 2009

PARI
```
a(n)=if(n<1,n==0,a(n\2)+a(n\4))
```

For n > 0, a(n) = F([log(n)/log(2)]+3) where F(k) denotes the k-th Fibonacci number. For n >= 3, F(n) appears 2^(n-3) times. More generally, if p is an integer > 1 and a(n) = a(floor(n/p)) + a(floor(n/p^2)), n > 0, a(0)=1, then for n > 0, a(n) = F(floor(log(n)/log(p)) + 3).

A088468 a(0) = 1, a(n) = a(floor(n/2)) + a(floor(n/3)) for n > 0.

Original entry on oeis.org

1, 2, 3, 4, 5, 5, 7, 7, 8, 9, 9, 9, 12, 12, 12, 12, 13, 13, 16, 16, 16, 16, 16, 16, 20, 20, 20, 21, 21, 21, 21, 21, 22, 22, 22, 22, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 33, 33, 33, 33, 33, 33, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 38, 38, 38, 38, 38, 38, 38, 38, 48

Offset: 0

Michael Somos, Oct 02 2003

nonn

Record values greater than 1 occur at 3-smooth numbers: A160519(n)=a(A003586(n)) and A160519(m)A003586(n). - Reinhard Zumkeller, May 16 2009

Reinhard Zumkeller, Table of n, a(n) for n = 0..10000
A. R. Lebeck, CPS 104: Homework #3.
Michael Penn, Erdős but simpler, Youtube video.

Equals A061984(n) + 1.

Cf. A007731, A083662, A165704, A165706. - Reinhard Zumkeller, Sep 26 2009

a[0]=1;a[n_]:=a[n]=a[Floor[n/2]]+a[Floor[n/3]];Array[a,75,0] (* Harvey P. Dale, Aug 23 2020 *)

PARI
```
a(n)=if(n<1,n==0,a(n\2)+a(n\3))
```

Limit_{n->oo} a(n)/n = 0, as proved in Michael Penn's Youtube video (see Links). Michael Penn states in the video that this is a simplification of a problem of Paul Erdős, where the original problem is to show that limit_{n->oo} b(n)/n = 12/log(432) for b(0) = 1, b(n) = b(floor(n/2)) + b(floor(n/3)) + b(floor(n/6)) for n > 0 ({b(n)} is the sequence A007731). - Jianing Song, Sep 27 2023

A007731 a(n) = a(floor(n/2)) + a(floor(n/3)) + a(floor(n/6)), with a(0) = 1.

Original entry on oeis.org

1, 3, 5, 7, 9, 9, 15, 15, 17, 19, 19, 19, 29, 29, 29, 29, 31, 31, 41, 41, 41, 41, 41, 41, 55, 55, 55, 57, 57, 57, 57, 57, 59, 59, 59, 59, 85, 85, 85, 85, 85, 85, 85, 85, 85, 85, 85, 85, 103, 103, 103, 103, 103, 103, 117, 117

Offset: 0

N. J. A. Sloane

T. D. Noe, Table of n, a(n) for n = 0..10000
P. Erdős, A. Hildebrand, A. Odlyzko, P. Pudaite and B. Reznick, The asymptotic behavior of a family of sequences, Pacific J. Math., 126 (1987), pp. 227-241.

Cf. A083662, A088468, A165704, A165706, A061984.

a007731 n = a007731_list !! n
a007731_list = 1 : (zipWith3 (\u v w -> u + v + w)
   (map (a007731 . (`div` 2)) [1..])
   (map (a007731 . (`div` 3)) [1..])
   (map (a007731 . (`div` 6)) [1..]))
-- Reinhard Zumkeller, Jan 11 2014

A007731 := proc(n) option remember; if n=0 then RETURN(1) else RETURN( A007731(trunc(n/2))+A007731(trunc(n/3))+A007731(trunc(n/6))); fi; end;
# second Maple program:
a:= proc(n) option remember; `if`(n=0, 1,
      add(a(floor(n/i)), i=[2, 3, 6]))
    end:
seq(a(n), n=0..100);  # Alois P. Heinz, Sep 27 2023

a[n_] := a[n] = a[Floor[n/2]] + a[Floor[n/3]] + a[Floor[n/6]] ; a[0] = 1; Table[a[n], {n, 0, 60}] (* Jean-François Alcover, Mar 06 2014 *)

a(n)=if(n<5, 2*n+1, a(n\2) + a(n\3) + a(n\6)) \\ Charles R Greathouse IV, Feb 08 2017

From given link, a(n) is asymptotic to c*n where c = 12/log(432) = 1.97744865... - Benoit Cloitre, Dec 18 2002

Name clarified by Michel Marcus, Apr 10 2025

A165706 a(0) = 1, a(n) = a([n/2]) + a([n/5]) for n > 1.

Original entry on oeis.org

1, 2, 3, 3, 4, 5, 5, 5, 6, 6, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 12, 12, 12, 12, 12, 13, 13, 13, 13, 13, 13, 13, 14, 14, 14, 14, 14, 14, 14, 14, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22

Offset: 0

Reinhard Zumkeller, Sep 26 2009

nonn

For n>0: A165707(n)=a(A003592(n)) and A165707(m)A003592(n).

R. Zumkeller, Table of n, a(n) for n = 0..10000

Cf. A007731, A083662, A088468, A165704.

a(n)=if(n<3, return(n+1)); a(n\2) + a(n\5) \\ Charles R Greathouse IV, Nov 14 2016

cp's OEIS Frontend

A165705 Range and record values of A165704.

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A051037 5-smooth numbers, i.e., numbers whose prime divisors are all <= 5.

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A083662 a(n) = a(floor(n/2)) + a(floor(n/4)), n > 0; a(0)=1.

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A088468 a(0) = 1, a(n) = a(floor(n/2)) + a(floor(n/3)) for n > 0.

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A007731 a(n) = a(floor(n/2)) + a(floor(n/3)) + a(floor(n/6)), with a(0) = 1.

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A165706 a(0) = 1, a(n) = a([n/2]) + a([n/5]) for n > 1.

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