A046306 -id:A046306 - cp's OEIS frontend

A120037 Number of 6-almost primes 6ap such that 2^n < 6ap <= 2^(n+1).

0, 0, 0, 0, 0, 1, 1, 5, 8, 22, 44, 96, 215, 439, 959, 1967, 4185, 8735, 18143, 37695, 77939, 161479, 332008, 684502, 1404867, 2882712, 5904454, 12078654, 24682057, 50375102, 102724466, 209250102, 425921989, 866187909, 1760280404, 3574740094

Offset: 0

Jonathan Vos Post and Robert G. Wilson v, Mar 21 2006

nonn

The partial sum equals the number of Pi_6(2^n).

			(2^6, 2^7] there is one semiprime, namely 96. 64 was counted in the previous entry.

Cf. A046306, A036378, A120033, A120034, A120035, A120036, A120037, A120038, A120039, A120040, A120041, A120042, A120043.

AlmostPrimePi[k_Integer, n_] := Module[{a, i}, a[0] = 1; If[k == 1, PrimePi[n], Sum[PrimePi[n/Times @@ Prime[Array[a, k - 1]]] - a[k - 1] + 1, Evaluate[ Sequence @@ Table[{a[i], a[i - 1], PrimePi[(n/Times @@ Prime[Array[a, i - 1]])^(1/(k - i + 1))]}, {i, k - 1}]]]]]; (* Eric W. Weisstein, Feb 07 2006 *)
t = Table[AlmostPrimePi[6, 2^n], {n, 0, 30}]; Rest@t - Most@t

A046305 Numbers that are divisible by at least 6 primes (counted with multiplicity).

Original entry on oeis.org

64, 96, 128, 144, 160, 192, 216, 224, 240, 256, 288, 320, 324, 336, 352, 360, 384, 400, 416, 432, 448, 480, 486, 504, 512, 528, 540, 544, 560, 576, 600, 608, 624, 640, 648, 672, 704, 720, 729, 736, 756, 768, 784, 792, 800, 810, 816, 832, 840, 864, 880, 896

Offset: 1

Patrick De Geest, Jun 15 1998

nonn

John Cerkan, Table of n, a(n) for n = 1..10000

Subsequence of A033987 and A046304.

Cf. A046306.

Select[Range[1000],Total[Transpose[FactorInteger[#]][[2]]]>5&]  (* Harvey P. Dale, Jan 13 2011 *)
Select[Range[1000],PrimeOmega[#]>5&] (* Harvey P. Dale, Apr 14 2019 *)

is(n)=bigomega(n)>5 \\ Charles R Greathouse IV, Sep 17 2015

from math import prod, isqrt
from sympy import primerange, primepi, integer_nthroot
def A046305(n):
    def bisection(f, kmin=0, kmax=1):
        while f(kmax) > kmax: kmax <<= 1
        kmin = kmax >> 1
        while kmax-kmin > 1:
            kmid = kmax+kmin>>1
            if f(kmid) <= kmid:
                kmax = kmid
            else:
                kmin = kmid
        return kmax
    def almostprimepi(n, k):
        if k==0: return int(n>=1)
        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, k)) if k>1 else primepi(n))
    def f(x): return n+1+sum(almostprimepi(x, k) for k in range(1, 6))
    return bisection(f, n, n) # Chai Wah Wu, Mar 29 2025

Product p_i^e_i with Sum e_i >= 6.

a(n) = n + O(n (log log n)^4/log n). - Charles R Greathouse IV, Apr 07 2017

Offset corrected by Andrew Howroyd, Aug 13 2024

A111346 Pierpont 6-almost primes. 6-almost primes of form (2^K)*(3^L)+1.

Original entry on oeis.org

14348908, 134217729, 1073741825, 139314069505, 231928233985, 264479053825, 282429536482, 618475290625, 705277476865, 3570467226625, 4398046511105, 8349416423425, 21134460321793, 35664401793025, 91507169819845

Offset: 1

Jonathan Vos Post, Nov 08 2005

			a(1) = 14348908 = (2^0)*(3^15)+1 = 2 * 2 * 7 * 31 * 61 * 271.
a(2) = 134217729 = (2^27)*(3^0)+1 = 3 * 3 * 3 * 3 * 19 * 87211.
a(3) = 1073741825 = (2^30)*(3^0)+1 = 5 * 5 * 13 * 41 * 61 * 1321.
a(4) = 139314069505 = (2^18)*(3^12)+1 = 5 * 13 * 17 * 61 * 337 * 6133.
a(100) = 151115727451828646838273 = (2^77)*(3^0)+1 = 3 * 43 * 617 * 683 * 78233 * 35532364099.
a(127) = 9671406556917033397649409 = (2^83)*(3^0)+1 = 3 * 499 * 1163 * 2657 * 155377 * 13455809771.
a(153) = 523347633027360537213511522 = (2^0)*(3^56)+1 = 2 * 17 * 113 * 193 * 19489 * 36214795668330833.
a(169) = 2475880078570760549798248449 = (2^91)*(3^0)+1 = 3 * 43 * 2731 * 224771 * 1210483 * 25829691707.

Eric Weisstein's World of Mathematics, Pierpont Prime
Eric Weisstein's World of Mathematics, Almost Prime

Intersection of A046306 and A055600.
A005109 gives the Pierpont primes, which are primes of the form (2^K)*(3^L)+1.
A113432 gives the Pierpont semiprimes, 2-almost primes of the form (2^K)*(3^L)+1.
A112797 gives the Pierpont 3-almost primes, of the form (2^K)*(3^L)+1.
A111344 gives the Pierpont 4-almost primes, of the form (2^K)*(3^L)+1.
A111345 gives the Pierpont 5-almost primes, of the form (2^K)*(3^L)+1.
A113739 gives the Pierpont 7-almost primes, of the form (2^K)*(3^L)+1.
A113740 gives the Pierpont 8-almost primes, of the form (2^K)*(3^L)+1.
A113741 gives the Pierpont 9-almost primes, of the form (2^K)*(3^L)+1.

list(lim)=my(v=List(), L=lim\1-1); for(e=0, logint(L, 3), my(t=3^e); while(t<=L, if(bigomega(t+1)==6, listput(v, t+1)); t*=2)); Set(v) \\ Charles R Greathouse IV, Feb 01 2017

a(n) is in this sequence iff there exist nonnegative integers K and L such that Omega((2^K)*(3^L)+1) = 6.

Extended by Ray Chandler, Nov 08 2005

A046332 Palindromes with exactly 6 prime factors (counted with multiplicity).

Original entry on oeis.org

2772, 2992, 6776, 8008, 21112, 21712, 21912, 23632, 23832, 25452, 25752, 25952, 27472, 28782, 29392, 40104, 40304, 40404, 42024, 42924, 44044, 44144, 44744, 44944, 45954, 46764, 46864, 48984, 53235, 54945, 55755, 59895, 60606, 61216

Offset: 1

Patrick De Geest, Jun 15 1998

Chai Wah Wu, Table of n, a(n) for n = 1..5000

Cf. A002113 (palindromes), A046306 (bigomega = 6), A046319.

Cf. A046396 (similar but terms must be squarefree), A373466 (similar, but only distinct prime divisors are counted).

N:= 6: # to get all terms of up to N digits
digrev:= proc(n) local L,Ln; L:= convert(n,base,10);Ln:= nops(L);
add(L[i]*10^(Ln-i),i=1..Ln);
end proc:
Res:= NULL:
for d from 2 to N do
  if d::even then
    m:= d/2;
    Res:= Res, select(numtheory:-bigomega=6,
           [seq](n*10^m + digrev(n), n=10^(m-1)..10^m-1));
  else
    m:= (d-1)/2;
    Res:= Res, select(numtheory:-bigomega=6,
           [seq](seq(n*10^(m+1)+y*10^m+digrev(n), y=0..9), n=10^(m-1)..10^m-1));
  fi
od:
map(op,[Res]); # Robert Israel, Dec 23 2014

A046332_upto(N, start=1, num_fact=6)={ my(L=List()); while(N >= start = nxt_A002113(start), bigomega(start)==num_fact && listput(L, start)); L} \\ M. F. Hasler, Jun 06 2024

from sympy import factorint
def palQgen10(l): # generator of palindromes in base 10 of length <= 2*l
    if l > 0:
        yield 0
        for x in range(1,l+1):
            for y in range(10**(x-1),10**x):
                s = str(y)
                yield int(s+s[-2::-1])
            for y in range(10**(x-1),10**x):
                s = str(y)
                yield int(s+s[::-1])
A046332_list = [x for x in palQgen10(4) if sum(list(factorint(x).values())) == 6]
# Chai Wah Wu, Dec 21 2014

Intersection of A002113 and A046306. - M. F. Hasler, Jun 06 2024

A114437 Indices of 6-almost prime triangular numbers.

Original entry on oeis.org

32, 48, 96, 99, 104, 111, 119, 120, 125, 152, 161, 168, 176, 188, 189, 195, 200, 208, 223, 231, 239, 240, 252, 260, 264, 275, 299, 300, 303, 304, 315, 336, 342, 343, 344, 352, 359, 363, 374, 377, 391, 392, 395, 400

Offset: 1

Jonathan Vos Post, Feb 14 2006

			a(1) = 48 because T(48) = TriangularNumber(48) = 48*(48+1)/2 = 1176 = 2^3 * 3 * 7^2 is a 6-almost prime.
a(2) = 96 because T(96) = 96*(96+1)/2 = 4656 = 2^4 * 3 * 97 is a 6-almost prime.
a(18) = 200 because T(200) = 200*(200+1)/2 = 20100 = 2^2 * 3 * 5^2 * 67 is a 6-almost prime.
a(29) = 300 because T(300) = 300*(300+1)/2 = 45150 = 2 * 3 * 5^2 * 7 * 43 is a 6-almost prime.
a(38) = 363 because T(363) = 363*(363+1)/2 = 45150 = 66066 = 2 * 3 * 7 * 11^2 * 13 is a 6-almost prime.

Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Eric Weisstein's World of Mathematics, Almost Prime.
Eric Weisstein's World of Mathematics, Triangular Number.

Cf. A000217, A001222, A046306, A069904.

Select[Range[400],PrimeOmega[(#(#+1))/2]==6&] (* Harvey P. Dale, Mar 29 2012 *)
Flatten[Position[Accumulate[Range[800]], ?(PrimeOmega[#]== 6 &)]] (* _Vincenzo Librandi, Apr 09 2014 *)

isA114437(n)=bigomega(n*(n+1)/2)==6 /* Michael B. Porter, Mar 30 2012 */

{a(n)} = {k such that A001222(A000217(k)) = 6}. {a(n)} = {k such that k*(k+1)/2 has exactly 6 prime factors, with multiplicity}.
{a(n)} = {k such that A000217(k) is an element of A046306}.
{ m :  A069904(m) = 6 }. - Alois P. Heinz, Aug 05 2019

Corrected by Harvey P. Dale, Mar 29 2012

A125149 a(n) is the least k such that the n-almost prime count is positive and equal to the (n-1)-almost prime count. a(0) = 1.

Original entry on oeis.org

1, 2, 10, 15495, 151165506066

Offset: 0

Jonathan Vos Post and Robert G. Wilson v, Jan 07 2007

Unlike any of the prime number races in which any particular form may lead or trail, this sequence demonstrates that although the count of numbers having k prime factors begins by trailing the count for k-1 prime factors, eventually they exchange positions in the race. This can be seen by looking at A126279 or A126280.
The fundamental theorem of arithmetic, or unique factorization theorem, states that every natural number greater than 1 either is itself a prime number, or can be written as a unique product of prime numbers. It had a proof sketched by Euclid, then corrected and completed in "Disquisitiones Arithmeticae" [Carl Friedrich Gauss, 1801]. It fails in many rings of algebraic integers [Ernst Kummer, 1843], a discovery initiating algebraic number theory. Counting the elements in the unique product of prime numbers classifies natural numbers into primes, semiprimes, 3-almost primes and so on. This sequence quantifies a previously undescribed structure to that classification.
We took the first k where the two relevant counts are the same. If instead we took the least k such that the n-almost prime count from k onwards exceeds the (n-1)-almost prime count, the sequence would begin: 3, 34, 15530, ... [see A180126].
The prime count and the semiprime count are identical for 1, 10, 15, 16, 22, 25, 29, 30, 33.
The semiprime count and the 3-almost prime count are identical for 1, 2, 3, 15495, 15496, 15497, 15498, 15508, 15524, 15525, 15529.
The numbers of 3-almost primes and 4-almost primes are equal at 151165506066 and 731 larger numbers, the last one being 151165607041. See A180126. - T. D. Noe, Aug 11 2010
Landau's asymptotic formula suggests that a(n) is about exp(exp(n-1)). - Charles R Greathouse IV, Mar 14 2011

			a(1) = 2 since 1 has no prime factors and 2 has one prime factor, therefore prime factor counts of 0 and 1 occur equally often in the first 2 integers.
a(2) = 10 since there are 4 primes {2, 3, 5 & 7} and 4 semiprimes {4, 6, 9 & 10} less than or equal to 10.
a(4) = 151165506066 since there are 32437255807 4-almost primes and 3-almost primes <= a(4).

Andrew Granville and Greg Martin, Prime Number Races, arXiv:math/0408319 [math.NT], Aug 24 2004.
Eric Weisstein's World of Mathematics, Fundamental Theorem of Arithmetic.
Eric Weisstein's World of Mathematics, Modular Prime Counting Function.
Eric Weisstein's World of Mathematics, Prime Factor.

Cf. A126279, A126280, A117526.
Sequences listing r-almost primes, that is, k such that A001222(k) = r: A000040 (r = 1), A001358 (r = 2), A014612 (r = 3), A014613 (r = 4), A014614 (r = 5), A046306 (r = 6), A046308 (r = 7), A046310 (r = 8), A046312 (r = 9), A046314 (r = 10), A069272 (r = 11), A069273 (r = 12), A069274 (r = 13), A069275 (r = 14), A069276 (r = 15), A069277 (r = 16), A069278 (r = 17), A069279 (r = 18), A069280 (r = 19), A069281 (r = 20).
Cf. A180126.

AlmostPrimePi[k_Integer, n_] := Module[{a, i}, a[0] = 1; If[k == 1, PrimePi[n], Sum[PrimePi[n/Times @@ Prime[Array[a, k - 1]]] - a[k - 1] + 1, Evaluate[ Sequence @@ Table[{a[i], a[i - 1], PrimePi[(n/Times @@ Prime[Array[a, i - 1]])^(1/(k - i + 1))]}, {i, k - 1}]]]]]; (* Eric W. Weisstein, Feb 07 2006 *)
f[n_] := Block[{k = 2^n}, While[AlmostPrimePi[n, k] < AlmostPrimePi[n - 1, k], k++ ]; k];

Changed 33 to 34 in a comment. - T. D. Noe, Aug 11 2010

Edited by Peter Munn, Dec 17 2022

A112316 Number of partitions of n into products of 6 primes.

Original entry on oeis.org

1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2

Offset: 0

Jonathan Vos Post and Ray Chandler, Sep 02 2005

			a(160) = 2 since 160 = 96+64.

Alois P. Heinz, Table of n, a(n) for n = 0..10000

Cf. A046306, A000607, A101049, A112313, A112314, A112315.

A146297 Integers which are not the sum of a 6-almost prime and a prime.

Original entry on oeis.org

2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 68, 70, 72, 73, 74, 76, 78, 79

Offset: 2

Donovan Johnson, Nov 05 2008

nonn

Largest known term is 446185740 (see b-file). No more terms < 10^9.

			68 is in this sequence because no 6-almost prime and a prime sum to 68. 69 is not in this sequence because the sum of 64 (6-almost prime) and 5 (prime) is 69.

Donovan Johnson, Table of n, a(n) for n=2..20150

Cf. A000040, A046306, A130588, A146295, A146296.

Complement[Range[1000], Union@Flatten@Outer[Plus, Select[Range[1000], PrimeOmega[#] == 6 &], Prime[Range[PrimePi[1000]]]]] (* Robert Price, Jun 16 2019 *)

A086052 Sum of first n 6-almost primes.

Original entry on oeis.org

64, 160, 304, 464, 680, 904, 1144, 1468, 1804, 2156, 2516, 2916, 3332, 3818, 4322, 4850, 5390, 5934, 6494, 7094, 7702, 8326, 9055, 9791, 10547, 11331, 12123, 12933, 13749, 14589, 15469, 16369, 17281, 18209, 19145, 20137, 21137, 22177, 23281

Offset: 1

Shyam Sunder Gupta, Aug 24 2003

The first two elements in this sequence are themselves 6-almost primes. a(1) = 64 = 2^6. a(2) = 160 = 2^5 * 5. - Jonathan Vos Post, Dec 11 2004

			a(2)=160 because sum of first two 6-almost primes, i.e. 64+96, is 160.

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

Partial sums of A046306.

ListTools:-PartialSums(select(numtheory:-bigomega=6, [$1..2000])); # Robert Israel, Jun 28 2019

Accumulate[Select[Range[1500],PrimeOmega[#]==6&]] (* Harvey P. Dale, May 15 2013 *)

A110893 Numbers with a semiprime number of prime divisors (counted with multiplicity).

Original entry on oeis.org

16, 24, 36, 40, 54, 56, 60, 64, 81, 84, 88, 90, 96, 100, 104, 126, 132, 135, 136, 140, 144, 150, 152, 156, 160, 184, 189, 196, 198, 204, 210, 216, 220, 224, 225, 228, 232, 234, 240, 248, 250, 260, 276, 294, 296, 297, 306, 308, 315, 324, 328, 330, 336, 340, 342

Offset: 1

Jonathan Vos Post, Sep 20 2005

Below 256 = 2^8 this is identical to A067028 (Numbers with a composite number of prime factors, counted with multiplicity).

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Cf. A000040, A001222, A001358, A014613, A046306, A046312, A046314, A063989, A067028, A069276.

is(n)=n>9 && bigomega(bigomega(n))==2 \\ Charles R Greathouse IV, Jan 31 2017

a(n) such that A001222(a(n)) is an element of A001358. a(n) such that bigomega(a(n)) is an element of A001358. Union[4-almost primes(A014613), 6-almost primes(A046306), 9-almost primes(A046312), 10-almost primes(A046314), 14-almost primes(A069275), 15-almost primes(A069276), 21-almost primes, 22-almost primes, 25-almost primes, 26-almost primes, ...]

cp's OEIS Frontend

A120037 Number of 6-almost primes 6ap such that 2^n < 6ap <= 2^(n+1).

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A046305 Numbers that are divisible by at least 6 primes (counted with multiplicity).

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A111346 Pierpont 6-almost primes. 6-almost primes of form (2^K)*(3^L)+1.

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A046332 Palindromes with exactly 6 prime factors (counted with multiplicity).

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A114437 Indices of 6-almost prime triangular numbers.

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A125149 a(n) is the least k such that the n-almost prime count is positive and equal to the (n-1)-almost prime count. a(0) = 1.

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A112316 Number of partitions of n into products of 6 primes.

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A146297 Integers which are not the sum of a 6-almost prime and a prime.

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