A014614 -id:A014614 - cp's OEIS frontend

A114436 Indices of 5-almost prime triangular numbers.

15, 24, 27, 31, 35, 39, 44, 47, 54, 55, 56, 71, 72, 75, 79, 81, 84, 87, 90, 98, 107, 108, 112, 116, 124, 132, 134, 140, 147, 153, 155, 162, 164, 167, 170, 171, 174, 179, 180, 183, 184, 199, 203, 204, 209, 219, 220, 225, 230, 234, 244, 245, 247, 248, 249

Offset: 1

Jonathan Vos Post, Feb 13 2006

			a(1) = 15 because T(15) = TriangularNumber(15) = 15*(15+1)/2 = 120 = 2^3 * 3 * 5 is a 5-almost prime.
a(2) = 24 because T(24) = 24*(24+1)/2 = 300 = 2^2 * 3 * 5^2 is a 5-almost prime.
a(3) = 27 because T(27) = 27*(27+1)/2 = 378 = 2 * 3^3 * 7 is a 5-almost prime.
a(4) = 31 because T(27) = 31*(31+1)/2 = 496 = 2^4 * 31 is a 5-almost prime.
a(17) = 84 because T(27) = 84*(84+1)/2 = 3570 = 2 * 3 * 5 * 7 * 17 is a 5-almost prime.

Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
Eric Weisstein's World of Mathematics, Triangular Number.
Eric Weisstein's World of Mathematics, Almost Prime.

Cf. A000217, A001222, A014614, A069904.

Select[Range[250],PrimeOmega[(#(#+1))/2]==5&] (* Harvey P. Dale, Sep 14 2012 *)
Flatten[Position[Accumulate[Range[700]], ?(PrimeOmega[#]== 5 &)]] (* _Vincenzo Librandi, Apr 09 2014 *)

{a(n)} = {k such that A001222(A000217(k)) = 5}. {a(n)} = {k such that k*(k+1)/2 has exactly 5 prime factors, with multiplicity}. {a(n)} = {k such that A000217(k) is an element of A014614}.

{ m : A069904(m) = 5 }. - Alois P. Heinz, Aug 05 2019

Corrected and extended by Harvey P. Dale, Apr 02 2011

A124309 5-almost primes indexed by primes.

Original entry on oeis.org

48, 72, 108, 120, 180, 208, 270, 280, 368, 420, 450, 520, 592, 612, 660, 700, 760, 828, 920, 952, 976, 1032, 1064, 1128, 1242, 1288, 1323, 1372, 1380, 1428, 1575, 1624, 1674, 1700, 1752, 1768, 1880, 1976, 2028, 2096, 2178, 2196, 2312, 2328, 2384, 2394, 2475

Offset: 1

Jonathan Vos Post, Oct 25 2006

			a(1) = 5almostprime(prime(1)) = 5almostprime(2) = 48 = 2^4 * 3.
a(2) = 5almostprime(prime(2)) = 5almostprime(3) = 72 = 2^3 * 3^2.
a(3) = 5almostprime(prime(3)) = 5almostprime(5) = 108 = 2^2 * 3^3.

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

Cf. A124308 Primes indexed by 5-almost primes. A124310 prime(5almostprime(n)) - 5almostprime(prime(n)). 4-almost primes indexed by primes = A124283. prime(4almostprime(n)) - 4almostprime(prime(n)) = A124284. Primes indexed by 3-almost primes = A124268. 3-almost primes indexed by primes = A124269. prime(3almostprime(n)) - 3almostprime(prime(n)) = A124270. See also A106349 Primes indexed by semiprimes. See also A106350 Semiprimes indexed by primes. See also A122824 Prime(semiprime(n)) - semiprime(prime(n)). Commutator [A000040, A001358] at n.

Cf. A000040, A014614, A114405, A114415, A124282-A124284, A124309.

list(lim)=my(v=List(),u=v); forprime(p=2,lim\16, forprime(q=2,min(lim\(8*p),p), forprime(r=2,min(lim\(4*p*q),q), forprime(s=2,min(lim\(2*p*q*r),r), forprime(t=2,min(lim\(p*q*r*s),s), listput(v,p*q*r*s*t)))))); v=Set(v); forprime(p=2,#v, listput(u,v[p])); v=0; Vec(u) \\ Charles R Greathouse IV, Feb 10 2017

a(n) = 5almostprime(prime(n)) = A014614(A000040(n)).

a(16)-a(47) from Giovanni Resta, Jun 13 2016

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

A086047 Sum of first n 5-almost primes.

Original entry on oeis.org

32, 80, 152, 232, 340, 452, 572, 734, 902, 1078, 1258, 1458, 1666, 1909, 2161, 2425, 2695, 2967, 3247, 3547, 3851, 4163, 4531, 4909, 5301, 5697, 6102, 6510, 6930, 7370, 7820, 8276, 8740, 9208, 9704, 10204, 10724, 11276, 11843, 12431, 13023, 13617

Offset: 1

Shyam Sunder Gupta, Aug 24 2003

Elements in this sequence can themselves be 5-almost primes. a(1) = 32 = 2^5. a(2) = 80 = 2^4 * 5. a(27) = 6102 = 2 * 3^3 * 113 a(28) = 6510 = 2 * 3 * 5 * 7 * 31 a(31) = 7820 = 2^2 * 5 * 17 * 23 a(33) = 8740 = 2^2 * 5 * 19 * 23. Does this happen infinitely often? - Jonathan Vos Post, Dec 11 2004

			a(2)=80 because sum of first two 5-almost primes, i.e. 32+48, is 80.

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

Partial sums of A014614.

Accumulate[Select[Range[1000],PrimeOmega[#]==5&]] (* Harvey P. Dale, Jan 19 2018 *)

A112315 Number of partitions of n into products of 5 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, 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, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 1, 0

Offset: 0

Jonathan Vos Post and Ray Chandler, Sep 02 2005

nonn

			a(96) = 2 since 96 = 48+48 = 32+32+32.

Alois P. Heinz, Table of n, a(n) for n = 0..5000

Cf. A014614, A000607, A101049, A112313, A112314, A112316.

A124308 Primes indexed by 5-almost primes.

Original entry on oeis.org

131, 223, 359, 409, 593, 613, 659, 953, 997, 1049, 1069, 1223, 1283, 1543, 1601, 1693, 1733, 1747, 1811, 1987, 2003, 2069, 2503, 2593, 2693, 2713, 2789, 2801, 2903, 3079, 3181, 3221, 3301, 3323, 3541, 3571, 3727, 4003, 4127, 4283

Offset: 1

Jonathan Vos Post, Oct 25 2006

			a(1) = prime(5almostprime(1)) = prime(32 = 2^5) = 131.
a(2) = prime(5almostprime(2)) = prime(48 = 2^4 * 3) = 223.
a(3) = prime(5almostprime(3)) = prime(72 = 2^3 * 3^2) = 359.
a(4) = prime(5almostprime(4)) = prime(80 = 2^4 * 5) = 409.

Cf. A124309 5-almost primes indexed by primes. A124310 prime(5almostprime(n)) - 5almostprime(prime(n)). 4-almost primes indexed by primes = A124283. prime(4almostprime(n)) - 4almostprime(prime(n)) = A124284. Primes indexed by 3-almost primes = A124268. 3-almost primes indexed by primes = A124269. prime(3almostprime(n)) - 3almostprime(prime(n)) = A124270. See also A106349 Primes indexed by semiprimes. See also A106350 Semiprimes indexed by primes. See also A122824 Prime(semiprime(n)) - semiprime(prime(n)). Commutator [A000040, A001358] at n.

Cf. A000040, A014614, A114405, A114415, A122824, A124269, A124270, A124282-A124284, A124309, A124310.

Prime[#]&/@Select[Range[600],PrimeOmega[#]==5&] (* Harvey P. Dale, Nov 20 2015 *)

a(n) = prime(5almostprime(n)) = A000040(A014614(n)). {p such that p is prime and omega(primepi(p)) = 5} = {p such that p is in A000040 and A001222(A000720(p)) = 5}.

A146296 Integers which are not the sum of a 5-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, 36, 38, 40, 41, 42, 44, 46, 47, 48, 52, 54, 56, 57, 58, 60, 62, 64, 66, 68, 70, 72, 76, 78, 80, 81, 84, 86, 88, 90, 92, 94, 96, 98, 100, 102, 104, 106, 108

Offset: 2

Donovan Johnson, Nov 05 2008

Largest term is 3573570 (see b-file). No more terms < 10^9. Conjectured to be complete.

			36 is in this sequence because no 5-almost prime and a prime sum to 36. 37 is not in this sequence because the sum of 32 (5-almost prime) and 5 (prime) is 37.

Donovan Johnson, Table of n, a(n) for n=2..2008

Cf. A000040, A014614, A130588, A146295, A146297.

Complement[Range[1000], Union@Flatten@Outer[Plus, Select[Range[1000], PrimeOmega[#] == 5 &], Prime[Range[PrimePi[1000]]]]] (* Robert Price, Jun 16 2019 *)

A180150 Numbers n such that n and n+2 are both divisible by exactly 4 primes (counted with multiplicity).

Original entry on oeis.org

54, 88, 150, 196, 232, 248, 294, 306, 328, 340, 342, 348, 460, 488, 490, 568, 570, 664, 712, 738, 774, 850, 856, 858, 868, 870, 948, 1012, 1014, 1060, 1096, 1110, 1148, 1190, 1204, 1206, 1208, 1210, 1218, 1254, 1274, 1276, 1290, 1302, 1314, 1420, 1430, 1448

Offset: 1

Jonathan Vos Post, Aug 12 2010

"Quadruprimes" or "4-almost primes" that keep that property when incremented by 2. This sequence is to 4 as 3 is to A180117, as A092207 is to 2, and as A001359 is to 1. That is, this sequence is the 4th row of the infinite array A[k,n] = n-th natural number m such that m and m+2 are both divisible by exactly k primes (counted with multiplicity). The first row is the lesser of twin primes. The second row is the sequence such that m and m+2 are both semiprimes.

			a(1) = 54 because 54 = 2 * 3^3 is divisible by exactly 4 primes (counted with multiplicity), and so is 54+2 = 56 = 2^3 * 7.

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

Cf. A000040, A001222, A001358, A014614, A033987, A101637, A114106 (number of 4-almost primes <= 10^n).

SequencePosition[PrimeOmega[Range[1500]],{4,,4}][[;;,1]] (* _Harvey P. Dale, Jan 14 2024 *)

is(n)=bigomega(n)==4 && bigomega(n+2)==4 \\ Charles R Greathouse IV, Jan 31 2017

{m in A014613 and m+2 in A014613} = {m such that bigomega(m) = bigomega(m+2) = 4} = {m such that A001222(m) = A001222(m+2) = 4}.

More terms from R. J. Mathar, Aug 13 2010

A207790 Permutation of positive numbers. See comments.

Original entry on oeis.org

1, 2, 4, 3, 8, 5, 6, 7, 16, 11, 9, 13, 12, 17, 10, 19, 32, 23, 14, 29, 18, 31, 15, 37, 24, 41, 21, 43, 20, 47, 22, 53, 64, 59, 25, 61, 27, 67, 26, 71, 36, 73, 33, 79, 28, 83, 34, 89, 48, 97, 35, 101, 30, 103, 38, 107, 40, 109, 39, 113, 42, 127, 46, 131, 128, 137, 49, 139, 44, 149, 51, 151

Offset: 1

Vladimir Shevelev and Peter J. C. Moses, Feb 20 2012

nonn

a(1)=1; on places 2,4,6,8,... we put primes (A000040); on places 3,7,11,15,... we put products of two primes (A001358); on places 5,13,21,29,... we put products of three primes (A014612); on places 9,25,41,57,... we put products of four primes (A014613); on places 17,49,81,... we put products of five primes (A014614); etc.
Primes with the index not exceeding n have density 1/2, semiprimes have density 1/4, etc.
By our system, here and in  A207800, A207801, A207802 we used the order: a(1)=1; the first appearance  of a new kind of numbers in places of the form 2^k+1, k=0,1,2,..., with  period of the appearance 2^{k+1}.

Ivan Neretin, Table of n, a(n) for n = 1..8192

Cf. A057114.

mx = 72; a = Array[1 &, mx]; cnt = mx - 1; offs = Table[2^(i - 1) + 1, {i, 1, mx}]; n = 1; While[cnt > 0, n++; idx = PrimeOmega[n]; pos = offs[[idx]]; If[pos > mx, Continue[]]; offs[[idx]] += 2^idx; a[[pos]] = n; cnt--]; a (* Ivan Neretin, May 06 2015 *)

For n>1, a(n) = A078840(A249725(n-1)). - Ivan Neretin, Apr 30 2016

A037088 Triangle read by rows: T(n,k) is number of numbers x, 2^n <= x < 2^(n+1), with k prime factors (counted with multiplicity).

Original entry on oeis.org

2, 2, 2, 2, 4, 2, 5, 4, 5, 2, 7, 12, 6, 5, 2, 13, 20, 17, 7, 5, 2, 23, 40, 30, 20, 8, 5, 2, 43, 75, 65, 37, 21, 8, 5, 2, 75, 147, 131, 81, 41, 22, 8, 5, 2, 137, 285, 257, 173, 91, 44, 22, 8, 5, 2, 255, 535, 536, 344, 199, 96, 46, 22, 8, 5, 2, 464, 1062, 1033, 736, 403, 215, 99, 47

Offset: 1

Alford Arnold

Sequence A092097 gives the limiting behavior at the end of the rows. - T. D. Noe, Feb 22 2008

			The triangular array begins 2; 2,2; 2,4,2; 5,4,5,2; 7,12,6,5,2; ...
a(7) = 5 because the 3-almost primes between 16 and 32 are (18,20,27,28,30).

T. D. Noe, Rows n=1..24 of triangle, flattened

A001222 counts factors of n. A000040, A001358, A014612-A014614 are special cases. A036378 and A025488 are applications of binary order A029837. Leading diagonal is essentially A036378 and has partial sums A007053.

t[n_, k_] := Count[Range[2^n, 2^(n+1)-1], x_ /; Total[FactorInteger[x][[All, 2]]] == k]; Table[t[n, k], {n, 1, 12}, {k, 1, n}] // Flatten (* Jean-François Alcover, Nov 07 2013 *)

More terms from Naohiro Nomoto, Jun 18 2001

cp's OEIS Frontend

A114436 Indices of 5-almost prime triangular numbers.

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A124309 5-almost primes indexed by primes.

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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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A086047 Sum of first n 5-almost primes.

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A112315 Number of partitions of n into products of 5 primes.

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A124308 Primes indexed by 5-almost primes.

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A146296 Integers which are not the sum of a 5-almost prime and a prime.

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A180150 Numbers n such that n and n+2 are both divisible by exactly 4 primes (counted with multiplicity).

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