Powerful number
A powerful number is a positive integer m such that for every prime number p dividing m, p^{2} also divides m. Equivalently, a powerful number is the product of a square and a cube, that is, a number m of the form m = a^{2}b^{3}, where a and b are positive integers. Powerful numbers are also known as squareful, squarefull, or 2full. Paul Erdős and George Szekeres studied such numbers and Solomon W. Golomb named such numbers powerful.
The following is a list of all powerful numbers between 1 and 1000:
 1, 4, 8, 9, 16, 25, 27, 32, 36, 49, 64, 72, 81, 100, 108, 121, 125, 128, 144, 169, 196, 200, 216, 225, 243, 256, 288, 289, 324, 343, 361, 392, 400, 432, 441, 484, 500, 512, 529, 576, 625, 648, 675, 676, 729, 784, 800, 841, 864, 900, 961, 968, 972, 1000, ... (sequence A001694 in the OEIS).
Equivalence of the two definitions
If m = a^{2}b^{3}, then every prime in the prime factorization of a appears in the prime factorization of m with an exponent of at least two, and every prime in the prime factorization of b appears in the prime factorization of m with an exponent of at least three; therefore, m is powerful.
In the other direction, suppose that m is powerful, with prime factorization
 [math]\displaystyle{ m = \prod p_i^{\alpha_i}, }[/math]
where each α_{i} ≥ 2. Define γ_{i} to be three if α_{i} is odd, and zero otherwise, and define β_{i} = α_{i} − γ_{i}. Then, all values β_{i} are nonnegative even integers, and all values γ_{i} are either zero or three, so
 [math]\displaystyle{ m = \left(\prod p_i^{\beta_i}\right)\left(\prod p_i^{\gamma_i}\right) = \left(\prod p_i^{\beta_i/2}\right)^2\left(\prod p_i^{\gamma_i/3}\right)^3 }[/math]
supplies the desired representation of m as a product of a square and a cube.
Informally, given the prime factorization of m, take b to be the product of the prime factors of m that have an odd exponent (if there are none, then take b to be 1). Because m is powerful, each prime factor with an odd exponent has an exponent that is at least 3, so m/b^{3} is an integer. In addition, each prime factor of m/b^{3} has an even exponent, so m/b^{3} is a perfect square, so call this a^{2}; then m = a^{2}b^{3}. For example:
 [math]\displaystyle{ m = 21600 = 2^5 \times 3^3 \times 5^2 \, , }[/math]
 [math]\displaystyle{ b = 2 \times 3 = 6 \, , }[/math]
 [math]\displaystyle{ a = \sqrt{\frac{m}{b^3}} = \sqrt{2^2 \times 5^2} = 10 \, , }[/math]
 [math]\displaystyle{ m = a^2b^3 = 10^2 \times 6^3 \, . }[/math]
The representation m = a^{2}b^{3} calculated in this way has the property that b is squarefree, and is uniquely defined by this property.
Mathematical properties
The sum of the reciprocals of the powerful numbers converges. The value of this sum may be written in several other ways, including as the infinite product
 [math]\displaystyle{ \prod_p\left(1+\frac{1}{p(p1)}\right)=\frac{\zeta(2)\zeta(3)}{\zeta(6)} = \frac{315}{2\pi^4}\zeta(3)=1.9435964368..., }[/math]
where p runs over all primes, ζ(s) denotes the Riemann zeta function, and ζ(3) is Apéry's constant.^{[1]} (sequence A082695 in the OEIS) More generally, the sum of the reciprocals of the sth powers of the powerful numbers (a Dirichlet series generating function) is equal to
 [math]\displaystyle{ \frac{\zeta(2s)\zeta(3s)}{\zeta(6s)} }[/math]
whenever it converges.
Let k(x) denote the number of powerful numbers in the interval [1,x]. Then k(x) is proportional to the square root of x. More precisely,
 [math]\displaystyle{ cx^{1/2}3x^{1/3}\le k(x) \le cx^{1/2}, c = \zeta(3/2)/\zeta(3) = 2.173 \ldots }[/math]
(Golomb, 1970).
The two smallest consecutive powerful numbers are 8 and 9. Since Pell's equation x^{2} − 8y^{2} = 1 has infinitely many integral solutions, there are infinitely many pairs of consecutive powerful numbers (Golomb, 1970); more generally, one can find consecutive powerful numbers by solving a similar Pell equation x^{2} − ny^{2} = ±1 for any perfect cube n. However, one of the two powerful numbers in a pair formed in this way must be a square. According to Guy, Erdős has asked whether there are infinitely many pairs of consecutive powerful numbers such as (23^{3}, 2^{3}3^{2}13^{2}) in which neither number in the pair is a square. (Walker 1976) showed that there are indeed infinitely many such pairs by showing that 3^{3}c^{2} + 1 = 7^{3}d^{2} has infinitely many solutions. Walker's solutions to this equation are generated, for any odd integer k, by considering the number
 [math]\displaystyle{ (2\sqrt{7}+3\sqrt{3})^{7k}=a\sqrt{7}+b\sqrt{3}, }[/math]
for integers a divisible by 7 and b divisible by 3, and constructing from a and b the consecutive powerful numbers 7a^{2} and 3b^{2} with 7a^{2} = 1 + 3b^{2}. The smallest consecutive pair in this family is generated for k = 1, a = 2637362, and b = 4028637 as
 [math]\displaystyle{ 7\cdot 2637362^2 = 2^2\cdot 7^3\cdot 13^2\cdot 43^2\cdot 337^2=48689748233308 }[/math]
and
 [math]\displaystyle{ 3\cdot 4028637^2 = 3^3\cdot 139^2\cdot 9661^2 = 48689748233307. }[/math]
Unsolved problem in mathematics: Can three consecutive numbers be powerful? (more unsolved problems in mathematics)

It is a conjecture of Erdős, Mollin, and Walsh that there are no three consecutive powerful numbers. If a triplet of consecutive powerful numbers exists, then its smallest term must be congruent to 7, 27, or 35 modulo 36.^{[2]}
Sums and differences of powerful numbers
Any odd number is a difference of two consecutive squares: (k + 1)^{2} = k^{2} + 2k + 1, so (k + 1)^{2} − k^{2} = 2k + 1. Similarly, any multiple of four is a difference of the squares of two numbers that differ by two: (k + 2)^{2} − k^{2} = 4k + 4. However, a singly even number, that is, a number divisible by two but not by four, cannot be expressed as a difference of squares. This motivates the question of determining which singly even numbers can be expressed as differences of powerful numbers. Golomb exhibited some representations of this type:
 2 = 3^{3} − 5^{2}
 10 = 13^{3} − 3^{7}
 18 = 19^{2} − 7^{3} = 3^{5} − 15^{2}.
It had been conjectured that 6 cannot be so represented, and Golomb conjectured that there are infinitely many integers which cannot be represented as a difference between two powerful numbers. However, Narkiewicz showed that 6 can be so represented in infinitely many ways such as
 6 = 5^{4}7^{3} − 463^{2},
and McDaniel showed that every integer has infinitely many such representations (McDaniel, 1982).
Erdős conjectured that every sufficiently large integer is a sum of at most three powerful numbers; this was proved by Roger HeathBrown (1987).
Generalization
More generally, we can consider the integers all of whose prime factors have exponents at least k. Such an integer is called a kpowerful number, kful number, or kfull number.
 (2^{k+1} − 1)^{k}, 2^{k}(2^{k+1} − 1)^{k}, (2^{k+1} − 1)^{k+1}
are kpowerful numbers in an arithmetic progression. Moreover, if a_{1}, a_{2}, ..., a_{s} are kpowerful in an arithmetic progression with common difference d, then
 a_{1}(a_{s} + d)^{k},
a_{2}(a_{s} + d)^{k}, ..., a_{s}(a_{s} + d)^{k}, (a_{s} + d)^{k+1}
are s + 1 kpowerful numbers in an arithmetic progression.
We have an identity involving kpowerful numbers:
 a^{k}(a^{l} + ... + 1)^{k} + a^{k + 1}(a^{l} + ... + 1)^{k} + ... + a^{k + l}(a^{l} + ... + 1)^{k} = a^{k}(a^{l} + ... +1)^{k+1}.
This gives infinitely many l+1tuples of kpowerful numbers whose sum is also kpowerful. Nitaj shows there are infinitely many solutions of x+y=z in relatively prime 3powerful numbers(Nitaj, 1995). Cohn constructs an infinite family of solutions of x+y=z in relatively prime noncube 3powerful numbers as follows: the triplet
 X = 9712247684771506604963490444281, Y = 32295800804958334401937923416351, Z = 27474621855216870941749052236511
is a solution of the equation 32X^{3} + 49Y^{3} = 81Z^{3}. We can construct another solution by setting X′ = X(49Y^{3} + 81Z^{3}), Y′ = −Y(32X^{3} + 81Z^{3}), Z′ = Z(32X^{3} − 49Y^{3}) and omitting the common divisor.
See also
Notes
 ↑ (Golomb, 1970)
 ↑ Beckon, Edward (2019). "On Consecutive Triples of Powerful Numbers". RoseHulman Undergraduate Mathematics Journal 20 (2): 25–27. https://scholar.rosehulman.edu/cgi/viewcontent.cgi?article=1424&context=rhumj.
References
 Cohn, J. H. E. (1998). "A conjecture of Erdős on 3powerful numbers". Math. Comp. 67 (221): 439–440. doi:10.1090/S0025571898008813. https://www.ams.org/mcom/199867221/S0025571898008813/.
 Erdős, Paul; Szekeres, George (1934). "Über die Anzahl der Abelschen Gruppen gegebener Ordnung und über ein verwandtes zahlentheoretisches Problem". Acta Litt. Sci. Szeged 7: 95–102.
 Golomb, Solomon W. (1970). "Powerful numbers". American Mathematical Monthly 77 (8): 848–852. doi:10.2307/2317020.
 Guy, Richard K. (2004). Unsolved Problems in Number Theory (3rd ed.). SpringerVerlag. pp. Section B16. ISBN 9780387208602.
 HeathBrown, Roger (1988). "Ternary quadratic forms and sums of three squarefull numbers". Boston: Birkhäuser. pp. 137–163.
 HeathBrown, Roger (1990). "Sums of three squarefull numbers". Colloq. Math. Soc. János Bolyai, no. 51. pp. 163–171.
 Ivić, Aleksandar (1985). The Riemann zetafunction. The theory of the Riemann zetafunction with applications. A WileyInterscience Publication. New York etc.: John Wiley & Sons. pp. 33–34,407–413. ISBN 9780471806349.
 McDaniel, Wayne L. (1982). "Representations of every integer as the difference of powerful numbers". Fibonacci Quarterly 20: 85–87.
 Nitaj, Abderrahmane (1995). "On a conjecture of Erdős on 3powerful numbers". Bull. London Math. Soc. 27 (4): 317–318. doi:10.1112/blms/27.4.317.
 Walker, David T. (1976). "Consecutive integer pairs of powerful numbers and related Diophantine equations". The Fibonacci Quarterly 14 (2): 111–116. https://www.fq.math.ca/Scanned/142/walker.pdf.
External links
 Powerfull number at Encyclopedia of Mathematics.
 Weisstein, Eric W.. "Powerful number". http://mathworld.wolfram.com/PowerfulNumber.html.
 The abc conjecture
 OEIS sequence A060355 (Numbers n such that n and n+1 are a pair of consecutive powerful numbers)
Original source: https://en.wikipedia.org/wiki/Powerful number.
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