A067003 -id:A067003 - cp's OEIS frontend

A081376 a(n) is the least number such that A067003[a(n)] = n.

1, 3, 4, 5, 7, 8, 9, 11, 13, 16, 17, 19, 23, 25, 27, 29, 31, 32, 37, 41, 43, 46, 48, 50, 51, 52, 54, 55, 56, 57, 58, 62, 63, 65, 68, 69, 72, 74, 75, 76, 77, 80, 82, 85, 86, 87, 88, 91, 92, 93, 94, 95, 96, 98, 99, 100, 104, 106, 108, 111, 112, 115, 116, 117, 118, 119, 122, 123

Offset: 1

Labos Elemer, Mar 24 2003

nonn

Cf. A067003, A081373-A081375, A001221.

g[x_] := Length[FactorInteger[x]] f[x_] := Count[Table[g[j] - g[x], {j, 1, x}], 0] Table[f[w], {w, 1, 100}]

A083399 Number of divisors of n that are not divisors of other divisors of n.

Original entry on oeis.org

1, 2, 2, 2, 2, 3, 2, 2, 2, 3, 2, 3, 2, 3, 3, 2, 2, 3, 2, 3, 3, 3, 2, 3, 2, 3, 2, 3, 2, 4, 2, 2, 3, 3, 3, 3, 2, 3, 3, 3, 2, 4, 2, 3, 3, 3, 2, 3, 2, 3, 3, 3, 2, 3, 3, 3, 3, 3, 2, 4, 2, 3, 3, 2, 3, 4, 2, 3, 3, 4, 2, 3, 2, 3, 3, 3, 3, 4, 2, 3, 2, 3, 2, 4, 3, 3, 3, 3, 2, 4, 3, 3, 3, 3, 3, 3, 2, 3, 3, 3, 2, 4

Offset: 1

Reinhard Zumkeller, Jun 12 2003

a(n) <= tau(n); a(n) = tau(n) iff n is prime or n=1 (A008578, A000040); a(n)=tau(n)-1 iff n is semiprime (A001358).
Number of noncomposite divisors of n, (cf. A008578). - Jaroslav Krizek, Nov 25 2009
From Wilf A. Wilson, Jul 21 2017: (Start)
a(n) is the number of maximal subsemigroups of the annular Jones monoid of degree n.
a(n) is the number of maximal subsemigroups of the monoid of orientation-preserving mappings on a set with n elements.
a(n) + 1 is the number of maximal subsemigroups of the monoid of orientation-preserving partial mappings on a set with n elements.
(End)
This is the restricted growth sequence transform of A001221 (and thus also of A007875, A034444, A082476, A292586 and many other sequences). This follows from the formula a(n) = 1+A001221(n), and from the fact that for any n, A001221(n) <= 1+A001221(k) for all k = 1..(n-1). A067003 gives the ordinal transform of A001221. See also A292582, A292583, A292585. - Antti Karttunen, Sep 25 2017

			{1,2,3,4,6,8,12,24} are the divisors of n=24: 1, 2, 3, 4 and 6 divide not only 24, but also 8 or 12, therefore a(24) = 3.
{1,2,3,4,6,8,12,24} are the divisors of n=24: 1, 2 and 3 are noncomposites, therefore a(24) = 3. - _Jaroslav Krizek_, Nov 25 2009

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
James East, Jitender Kumar, James D. Mitchell, and Wilf A. Wilson, Maximal subsemigroups of finite transformation and partition monoids, arXiv:1706.04967 [math.GR], 2017. [_Wilf A. Wilson_, Jul 21 2017]

Cf. A000005, A001221, A067003, A055212, A077761.

a083399 = (+ 1) . a001221  -- Reinhard Zumkeller, Sep 14 2015

[(#(PrimeDivisors(n)))+1: n in [1..100]]; // Vincenzo Librandi, Feb 15 2015

A083399 := proc(n)
    1+nops(numtheory[factorset](n)) ;
end proc:
seq(A083399(n),n=1..100) ; # R. J. Mathar, Sep 26 2017

A083399[n_Integer]:=1+PrimeNu[n]; (* Enrique Pérez Herrero, Jun 25 2010 *)
Rest@ CoefficientList[ Series[(1/(1 - x)) + Sum[1/(1 - x^Prime[j]), {j, 200}], {x, 0, 111}], x] (* Robert G. Wilson v, Aug 16 2011 *)

a(n)=#factor(n)~+1 \\ Charles R Greathouse IV, Sep 14 2015

a(n) = omega(n) + 1, where omega = A001221.
a(n) = tau(n) - A055212(n) = A000005(n)-A055212(n).
a(n) = A000005(n) - A033273(n) + 1. - Jaroslav Krizek, Nov 25 2009
a(n) = A010553(A007947(n)) = A000005(A000005(A007947(n))) = tau_2(tau_2(rad(n))). - Enrique Pérez Herrero, Jun 25 2010
G.f.: x/(1 - x) + Sum_{k>=1} x^prime(k)/(1 - x^prime(k)). - Ilya Gutkovskiy, Mar 21 2017
Sum_{k=1..n} a(k) = n * (log(log(n)) + B + 1) + O(n/log(n)), where B is Mertens's constant (A077761). - Amiram Eldar, Sep 29 2024

A058933 Let k be bigomega(n) (i.e., n is a k-almost-prime). a(n) = number of k-almost-primes <= n.

Original entry on oeis.org

1, 1, 2, 1, 3, 2, 4, 1, 3, 4, 5, 2, 6, 5, 6, 1, 7, 3, 8, 4, 7, 8, 9, 2, 9, 10, 5, 6, 10, 7, 11, 1, 11, 12, 13, 3, 12, 14, 15, 4, 13, 8, 14, 9, 10, 16, 15, 2, 17, 11, 18, 12, 16, 5, 19, 6, 20, 21, 17, 7, 18, 22, 13, 1, 23, 14, 19, 15, 24, 16, 20, 3, 21, 25, 17, 18, 26, 19, 22, 4, 8, 27, 23

Offset: 1

Naohiro Nomoto, Jan 11 2001

Equivalently, the number of positive integers less than or equal to n with the same number of prime factors as n, counted with multiplicity. - Gus Wiseman, Dec 28 2018

There is a close relationship between a(n) and a(n^2). See A209934 for an exploratory quantification. - Peter Munn, Aug 04 2019

			3 is prime, so a(3)=2. 10 is 2-almost prime (semiprime), so a(10)=4.
From _Gus Wiseman_, Dec 28 2018: (Start)
Column n lists the a(n) positive integers less than or equal to n with the same number of prime factors as n, counted with multiplicity:
  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20
  ---------------------------------------------------------------------
  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20
        2     3  4  5     6  9   7   8   11  10  14      13  12  17  18
              2     3     4  6   5       7   9   10      11  8   13  12
                    2        4   3       5   6   9       7       11  8
                                 2       3   4   6       5       7
                                         2       4       3       5
                                                         2       3
                                                                 2
(End)

Alois P. Heinz, Table of n, a(n) for n = 1..20000

Positions of 1's are A000079.
Cf. A001358, A014612, A014613, A014614.
Cf. A000010, A000961, A001222, A006049, A045920, A061142, A067003, A078843, A209934, A302242, A322838, A322839, A322840.
Equivalent sequence restricted to squarefree numbers: A340313.

p:= proc() 0 end:
a:= proc(n) option remember; local t;
      t:= numtheory[bigomega](n);
      p(t):= p(t)+1
    end:
seq(a(n), n=1..100);  # Alois P. Heinz, Oct 09 2015

p[] = 0; a[n] := a[n] = Module[{t}, t = PrimeOmega[n]; p[t] = p[t]+1]; Table[a[n], {n, 1, 100}] (* Jean-François Alcover, Jan 24 2017, after Alois P. Heinz *)

a(n) = my(k=bigomega(n)); sum(i=1, n, bigomega(i)==k); \\ Michel Marcus, Jun 27 2024

from math import prod, isqrt
from sympy import isprime, primepi, primerange, integer_nthroot, primeomega
def A058933(n):
    if n==1: return 1
    if isprime(n): return primepi(n)
    def g(x,a,b,c,m): yield from (((d,) for d in enumerate(primerange(b,isqrt(x//c)+1),a)) if m==2 else (((a2,b2),)+d for a2,b2 in enumerate(primerange(b,integer_nthroot(x//c,m)[0]+1),a) for d in g(x,a2,b2,c*b2,m-1)))
    return int(sum(primepi(n//prod(c[1] for c in a))-a[-1][0] for a in g(n,0,1,1,primeomega(n)))) # Chai Wah Wu, Aug 28 2024

Ordinal transform of A001222 (bigomega). - Franklin T. Adams-Watters, Aug 28 2006

If a(n) < a(3^A001222(2n)) = A078843(A001222(2n)) then a(2n) = a(n), otherwise a(2n) > a(n). - Peter Munn, Aug 05 2019

Name edited by Peter Munn, Dec 30 2022

A067004 Number of numbers <= n with same number of divisors as n.

Original entry on oeis.org

1, 1, 2, 1, 3, 1, 4, 2, 2, 3, 5, 1, 6, 4, 5, 1, 7, 2, 8, 3, 6, 7, 9, 1, 3, 8, 9, 4, 10, 2, 11, 5, 10, 11, 12, 1, 12, 13, 14, 3, 13, 4, 14, 6, 7, 15, 15, 1, 4, 8, 16, 9, 16, 5, 17, 6, 18, 19, 17, 1, 18, 20, 10, 1, 21, 7, 19, 11, 22, 8, 20, 2, 21, 23, 12, 13, 24, 9, 22, 2, 2, 25, 23, 3, 26, 27

Offset: 1

Henry Bottomley, Dec 21 2001

nonn

			a(10)=3 since 6,8,10 each have four divisors. a(11)=5 since 2,3,5,7,11 each have two divisors.

Paul Tek, Table of n, a(n) for n = 1..10000

Cf. A000005, A008479, A058933, A067003. Inverse of A000040, A001248, A030513, A030514, A030515, A030516, A030626, A030627, A030628, A030629, A030630, A030631, A030632, A030633, A030634, A030635, A030636, A030637, A030638 etc.
Cf. also A079788, A138009.
A047983(n) = a(n)-1.

N:= 1000: # to get a(1) to a(N)
R:= Vector(N):
for n from 1 to N do
  v:= numtheory:-tau(n);
  R[v]:= R[v]+1;
  A[n]:= R[v];
od:
seq(A[n],n=1..N); # Robert Israel, May 04 2015

b[_] = 0;
a[n_] := a[n] = With[{t = DivisorSigma[0, n]}, b[t] = b[t]+1];
Array[a, 105] (* Jean-François Alcover, Dec 20 2021 *)

a(n)=my(d=numdiv(n)); sum(k=1,n,numdiv(k)==d) \\ Charles R Greathouse IV, Sep 02 2015

Ordinal transform of A000005. - Franklin T. Adams-Watters, Aug 28 2006

a(A000040(n)^(p-1)) = n if p is prime. - Robert Israel, May 04 2015

A322838 Number of positive integers less than n with more prime factors than n, counted with multiplicity.

Original entry on oeis.org

0, 0, 0, 0, 1, 0, 2, 0, 1, 1, 5, 0, 6, 2, 2, 0, 9, 1, 10, 1, 5, 5, 13, 0, 6, 6, 2, 2, 18, 2, 19, 0, 10, 10, 10, 1, 24, 11, 11, 1, 27, 5, 28, 5, 5, 15, 31, 0, 16, 6, 17, 6, 36, 2, 19, 2, 20, 20, 41, 2, 42, 21, 9, 0, 23, 10, 47, 10, 25, 10, 50, 1, 51, 27, 11, 11

Offset: 1

Gus Wiseman, Dec 28 2018

nonn

			Column n lists the a(n) positive integers less than n with more prime factors than n:
  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20
  ---------------------------------------------------------------------
              4     6     8  8   10      12  12  12      16  16  18  16
                    4            9       10  8   8       15      16
                                 8       9               14      15
                                 6       8               12      14
                                 4       6               10      12
                                         4               9       10
                                                         8       9
                                                         6       8
                                                         4       6
                                                                 4

Positions of zeros appear to be A029744.

Cf. A000010, A000961, A001221, A001222, A045920, A058933, A061142, A067003, A302242, A322839, A322841.

Table[Length[Select[Range[n],PrimeOmega[#]>PrimeOmega[n]&]],{n,100}]

A334655 Number of integers less than n with the same number of distinct prime factors as n.

Original entry on oeis.org

0, 0, 1, 2, 3, 0, 4, 5, 6, 1, 7, 2, 8, 3, 4, 9, 10, 5, 11, 6, 7, 8, 12, 9, 13, 10, 14, 11, 15, 0, 16, 17, 12, 13, 14, 15, 18, 16, 17, 18, 19, 1, 20, 19, 20, 21, 21, 22, 22, 23, 24, 25, 23, 26, 27, 28, 29, 30, 24, 2, 25, 31, 32, 26, 33, 3, 27, 34, 35, 4, 28, 36, 29, 37, 38, 39, 40, 5, 30, 41

Offset: 1

Ilya Gutkovskiy, Oct 31 2020

nonn

			a(12) = 2 because omega(12) = 2 and also omega(6) = omega(10) = 2.

Robert Israel, Table of n, a(n) for n = 1..10000

Cf. A001221, A002110 (positions of 0's), A047983, A067003, A067004, A322837, A322841, A335097.

R:= NULL:
for n from 1 to 100 do
  w:= nops(numtheory:-factorset(n));
  if assigned(V[w]) then V[w]:= V[w]+1 else V[w]:= 1 fi;
  R:= R, V[w]-1
od:
R; # Robert Israel, Feb 25 2024

Table[Length[Select[Range[n - 1], PrimeNu[#] == PrimeNu[n] &]], {n, 80}]

a(n)={my(t=omega(n)); sum(k=1, n-1, omega(k)==t)} \\ Andrew Howroyd, Oct 31 2020

a(n) = |{j < n : omega(j) = omega(n)}|.

a(n) = A067003(n) - 1.

A322837 Number of positive integers less than n with fewer distinct prime factors than n.

Original entry on oeis.org

0, 1, 1, 1, 1, 5, 1, 1, 1, 8, 1, 9, 1, 10, 10, 1, 1, 12, 1, 13, 13, 13, 1, 14, 1, 15, 1, 16, 1, 29, 1, 1, 19, 19, 19, 19, 1, 20, 20, 20, 1, 40, 1, 22, 22, 22, 1, 23, 1, 24, 24, 24, 1, 25, 25, 25, 25, 25, 1, 57, 1, 27, 27, 1, 28, 62, 1, 29, 29, 65, 1, 30, 1, 31

Offset: 1

Gus Wiseman, Dec 28 2018

			Column n lists the a(n) positive integers less than n with fewer distinct prime factors than n:
  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20
  ---------------------------------------------------------------------
     1  1  1  1  5  1  1  1  9   1   11  1   13  13  1   1   17  1   19
                 4           8       9       11  11          16      17
                 3           7       8       9   9           13      16
                 2           5       7       8   8           11      13
                 1           4       5       7   7           9       11
                             3       4       5   5           8       9
                             2       3       4   4           7       8
                             1       2       3   3           5       7
                                     1       2   2           4       5
                                             1   1           3       4
                                                             2       3
                                                             1       2
                                                                     1

David A. Corneth, Table of n, a(n) for n = 1..10000
David A. Corneth, PARI program

Positions of 1's are A246655.

Cf. A000010, A001221, A006049, A067003, A294277, A294278, A302242, A322838, A322841.

Table[Length[Select[Range[n],PrimeNu[#]

PARI
```
\\ See Corneth link
```

A322841 Number of positive integers less than n with more distinct prime factors than n.

Original entry on oeis.org

0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 2, 0, 3, 0, 0, 5, 5, 0, 6, 0, 0, 0, 9, 0, 10, 0, 11, 0, 12, 0, 13, 13, 1, 1, 1, 1, 17, 1, 1, 1, 20, 0, 21, 2, 2, 2, 24, 2, 25, 2, 2, 2, 28, 2, 2, 2, 2, 2, 33, 0, 34, 3, 3, 36, 3, 0, 38, 4, 4, 0, 41, 5, 42, 5, 5, 5, 5, 0, 47, 6, 48

Offset: 1

Gus Wiseman, Dec 28 2018

nonn

			Column n lists the a(n) positive integers less than n with more distinct prime factors than n:
  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20
  ---------------------------------------------------------------------
                    6  6  6      10      12          15  15      18
                                  6      10          14  14      15
                                          6          12  12      14
                                                     10  10      12
                                                      6   6      10
                                                                  6

Alois P. Heinz, Table of n, a(n) for n = 1..20000

Positions of zeros are A116998.

Cf. A000010, A000961, A001221, A006049, A058933, A067003, A294277, A302242, A322837, A322838.

b:= proc(n) option remember; nops(numtheory[factorset](n)) end:
a:= proc(n) option remember;
      (t-> add(`if`(b(i)>t, 1, 0), i=1..n-1))(b(n))
    end:
seq(a(n), n=1..100);  # Alois P. Heinz, Dec 28 2018

Table[Length[Select[Range[n],PrimeNu[#]>PrimeNu[n]&]],{n,100}]

a(n) = my(omegan=omega(n)); sum(k=1, n-1, omega(k) > omegan); \\ Michel Marcus, Dec 29 2018

first(n) = {my(t = 1, pp = 1, res = vector(n)); forprime(p = 2, oo, pp*=p; if(pp > n, v = vector(t); break); t++); for(i = 1, n, o = omega(i); res[i] = v[o+1]; for(j = 1, o, v[j]++)); res} \\ David A. Corneth, Dec 29 2018

A340313 The n-th squarefree number is the a(n)-th squarefree number having its number of primes.

Original entry on oeis.org

1, 1, 2, 3, 1, 4, 2, 5, 6, 3, 4, 7, 8, 5, 6, 9, 7, 10, 1, 11, 8, 9, 10, 12, 11, 12, 13, 2, 14, 13, 15, 14, 16, 15, 16, 17, 17, 18, 18, 19, 3, 19, 20, 4, 20, 21, 21, 22, 5, 22, 23, 23, 24, 25, 26, 24, 27, 28, 29, 30, 25, 26, 6, 27, 7, 31, 28, 29, 8, 32, 30, 9

Offset: 1

Peter Dolland, Jan 04 2021

nonn

The sequence gives the column index of A005117(n) in the array A340316 and may be understood as a complementary addition to A072047 giving the row index.

			{x|x <= 6, A072047(x) = A072047(6) = 1} = {2,3,4,6}, therefore a(6) = 4.
{x|x <= 28, A072047(x) = A072047(28) = 3} = {19,28}, therefore a(28) = 2.

David A. Corneth, Table of n, a(n) for n = 1..10000
Alois P. Heinz, Plot of n, a(n) for n = 1..1000000

Cf. A001221, A001222, A005117 (squarefree numbers), A058933, A067003, A072047 (number of prime factors), A340316 (squarefree numbers array).

a340313 n = a340313_list !! (n-1)
a340313_list = repetitions a072047_list
    where
    repetitions [] = []
    repetitions (a:as) = 1 : h a as (repetitions as)
    h  []  = []
    h b (c:cs) (r:rs) = (if c == b then succ else id) r : h b cs rs

with(numtheory):
b:= proc(n) option remember; local k; if n=1 then 1 else
      for k from 1+b(n-1) while not issqrfree(k) do od; k fi
    end:
p:= proc() 0 end:
a:= proc(n) option remember; local h; a(n-1);
      h:= bigomega(b(n)); p(h):= p(h)+1;
    end: a(0):=0:
seq(a(n), n=1..100);  # Alois P. Heinz, Jan 06 2021

b[n_] := b[n] = Module[{k}, If[n == 1, 1,
     For[k = 1 + b[n - 1], !SquareFreeQ[k], k++]; k]];
p[_] = 0;
a[n_] := a[n] = Module[{h}, a[n - 1];
     h = PrimeOmega[b[n]]; p[h] = p[h]+1];
a[0] = 0;
Table[a[n], {n, 1, 100}] (* Jean-François Alcover, Mar 28 2022, after Alois P. Heinz *)

first(n) = {v = vector(5); n--; res = vector(n); t = 0; for(i = 2, oo, f = factor(i)[,2]; if(vecmax(f) == 1, if(#f > #v, v = concat(v, vector(#f - #v)) ); t++; v[#f]++; res[t] = v[#f]; if(t >= n, return(concat(1, res)) ) ) ) } \\ David A. Corneth, Jan 07 2021

from math import isqrt, prod
from sympy import primerange, integer_nthroot, mobius, primenu, primepi
def A340313(n):
    if n == 1: return 1
    def g(x,a,b,c,m): yield from (((d,) for d in enumerate(primerange(b+1,isqrt(x//c)+1),a+1)) if m==2 else (((a2,b2),)+d for a2,b2 in enumerate(primerange(b+1,integer_nthroot(x//c,m)[0]+1),a+1) for d in g(x,a2,b2,c*b2,m-1)))
    def f(x): return n+x-sum(mobius(k)*(x//k**2) for k in range(1, isqrt(x)+1))
    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
    kmax = bisection(f)
    return int(sum(primepi(kmax//prod(c[1] for c in a))-a[-1][0] for a in g(kmax,0,1,1,m)) if (m:=primenu(kmax)) > 1 else primepi(kmax)) # Chai Wah Wu, Aug 31 2024

a(n) = #{x|x <= n, A072047(x) = A072047(n)}.

A380654 Number of positive integers less than or equal to n that have the same sum of distinct prime factors as n.

Original entry on oeis.org

1, 1, 1, 2, 1, 2, 1, 3, 2, 2, 1, 3, 1, 1, 1, 4, 1, 4, 1, 3, 1, 2, 1, 5, 6, 1, 3, 2, 1, 2, 1, 5, 1, 2, 1, 7, 1, 1, 1, 4, 1, 2, 1, 3, 2, 1, 1, 8, 5, 6, 1, 2, 1, 9, 2, 3, 1, 2, 1, 3, 1, 1, 4, 6, 1, 3, 1, 3, 1, 2, 1, 10, 1, 1, 3, 2, 2, 3, 1, 7, 4, 2, 1, 3, 2, 1, 1, 4, 1, 5, 2, 2, 1, 1, 1, 11, 1, 4, 3, 8

Offset: 1

Ilya Gutkovskiy, Jan 29 2025

nonn

Ordinal transform of A008472.

Eric Weisstein's World of Mathematics, Sum of Prime Factors.

Cf. A008472, A067003, A263025, A380653.

b:= n-> add(i[1], i=ifactors(n)[2]):
p:= proc() 0 end:
a:= proc(n) option remember; local t;
      t:= b(n); p(t):= p(t)+1
    end:
seq(a(n), n=1..100);  # Alois P. Heinz, Jan 30 2025

sopf[n_] := DivisorSum[n, # &, PrimeQ[#] &]; Table[Length[Select[Range[n], sopf[#] == sopf[n] &]], {n, 1, 100}]

from sympy import factorint
from collections import Counter
from itertools import count, islice
def agen(): # generator of terms
    sopfcount = Counter()
    for n in count(1):
        key = sum(p for p in factorint(n))
        sopfcount[key] += 1
        yield sopfcount[key]
print(list(islice(agen(), 100))) # Michael S. Branicky, Jan 30 2025

a(n) = |{j <= n : sopf(j) = sopf(n)}|.

cp's OEIS Frontend

A081376 a(n) is the least number such that A067003[a(n)] = n.

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A083399 Number of divisors of n that are not divisors of other divisors of n.

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A058933 Let k be bigomega(n) (i.e., n is a k-almost-prime). a(n) = number of k-almost-primes <= n.

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A067004 Number of numbers <= n with same number of divisors as n.

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A322838 Number of positive integers less than n with more prime factors than n, counted with multiplicity.

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A334655 Number of integers less than n with the same number of distinct prime factors as n.

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Maple

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A322837 Number of positive integers less than n with fewer distinct prime factors than n.

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