If sum of two positive numbers is k, then sum of their cubes is minimum when they are

In mathematics and statistics, sums of powers occur in a number of contexts:

• Sums of squares arise in many contexts. For example, in geometry, the Pythagorean theorem involves the sum of two squares; in number theory, there are Legendre's three-square theorem and Jacobi's four-square theorem; and in statistics, the analysis of variance involves summing the squares of quantities.
• Faulhaber's formula expresses 1 k + 2 k + 3 k + ⋯ + n k {\displaystyle 1^{k}+2^{k}+3^{k}+\cdots +n^{k}}
  
  as a polynomial in n, or alternatively in term of a Bernoulli polynomial.
• Fermat's right triangle theorem states that there is no solution in positive integers for a 2 = b 4 + c 4 {\displaystyle a^{2}=b^{4}+c^{4}}
  
  and a 4 = b 4 + c 2 {\displaystyle a^{4}=b^{4}+c^{2}}
  
  .
• Fermat's Last Theorem states that x k + y k = z k {\displaystyle x^{k}+y^{k}=z^{k}}
  
  is impossible in positive integers with k>2.
• The equation of a superellipse is | x / a | k + | y / b | k = 1 {\displaystyle |x/a|^{k}+|y/b|^{k}=1}
  
  . The squircle is the case k = 4 , a = b {\displaystyle k=4,a=b}
  
  .
• Euler's sum of powers conjecture (disproved) concerns situations in which the sum of n integers, each a kth power of an integer, equals another kth power.
• The Fermat-Catalan conjecture asks whether there are an infinitude of examples in which the sum of two coprime integers, each a power of an integer, with the powers not necessarily equal, can equal another integer that is a power, with the reciprocals of the three powers summing to less than 1.
• Beal's conjecture concerns the question of whether the sum of two coprime integers, each a power greater than 2 of an integer, with the powers not necessarily equal, can equal another integer that is a power greater than 2.
• The Jacobi–Madden equation is a 4 + b 4 + c 4 + d 4 = ( a + b + c + d ) 4 {\displaystyle a^{4}+b^{4}+c^{4}+d^{4}=(a+b+c+d)^{4}}
  
  in integers.
• The Prouhet–Tarry–Escott problem considers sums of two sets of kth powers of integers that are equal for multiple values of k.
• A taxicab number is the smallest integer that can be expressed as a sum of two positive third powers in n distinct ways.
• The Riemann zeta function is the sum of the reciprocals of the positive integers each raised to the power s, where s is a complex number whose real part is greater than 1.
• The Lander, Parkin, and Selfridge conjecture concerns the minimal value of m + n in ∑ i = 1 n a i k = ∑ j = 1 m b j k . {\displaystyle \sum _{i=1}^{n}a_{i}^{k}=\sum _{j=1}^{m}b_{j}^{k}.}
  
• Waring's problem asks whether for every natural number k there exists an associated positive integer s such that every natural number is the sum of at most s kth powers of natural numbers.
• The successive powers of the golden ratio φ obey the Fibonacci recurrence:

φ n + 1 = φ n + φ n − 1 . {\displaystyle \varphi ^{n+1}=\varphi ^{n}+\varphi ^{n-1}.}

• Newton's identities express the sum of the kth powers of all the roots of a polynomial in terms of the coefficients in the polynomial.
• The sum of cubes of numbers in arithmetic progression is sometimes another cube.
• The Fermat cubic, in which the sum of three cubes equals another cube, has a general solution.
• The power sum symmetric polynomial is a building block for symmetric polynomials.
• The sum of the reciprocals of all perfect powers including duplicates (but not including 1) equals 1.
• The Erdős–Moser equation, 1 k + 2 k + ⋯ + m k = ( m + 1 ) k {\displaystyle 1^{k}+2^{k}+\cdots +m^{k}=(m+1)^{k}}
  
  where m {\displaystyle m}
  
  and k {\displaystyle k}
  
  are positive integers, is conjectured to have no solutions other than 11 + 21 = 31.
• The sums of three cubes cannot equal 4 or 5 modulo 9, but it is unknown whether all remaining integers can be expressed in this form.
• The sums of powers Sm(z, n) = zm + (z+1)m + ... + (z+n−1)m is related to the Bernoulli polynomials Bm(z) by (∂n−∂z) Sm(z, n) = Bm(z) and (∂2λ−∂Z) S2k+1(z, n) = Ŝ′k+1(Z) where Z = z(z−1), λ = S1(z, n), Ŝk+1(Z) ≡ S2k+1(0, z).[citation needed]
• The sum of the terms in the geometric series is ∑ k = i n z k = z i − z n + 1 1 − z . {\displaystyle \sum _{k=i}^{n}z^{k}={\frac {z^{i}-z^{n+1}}{1-z}}.}