identidade de Lagrange

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In algebra, Lagrange's identity, named after Joseph Louis Lagrange, is: ( ∑ k = 1 n a k 2 ) ( ∑ k = 1 n b k 2 ) − ( ∑ k = 1 n a k b k ) 2 = ∑ i = 1 n − 1 ∑ j = i + 1 n ( a i b j − a j b i ) 2 ( = 1 2 ∑ i = 1 n ∑ j = 1 , j ≠ i n ( a i b j − a j b i ) 2 ) , {\displaystyle {\begin{aligned}\left(\sum _{k=1}^{n}a_{k}^{2}\right)\left(\sum _{k=1}^{n}b_{k}^{2}\right)-\left(\sum _{k=1}^{n}a_{k}b_{k}\right)^{2}&=\sum _{i=1}^{n-1}\sum _{j=i+1}^{n}\left(a_{i}b_{j}-a_{j}b_{i}\right)^{2}\\&\left(={\frac {1}{2}}\sum _{i=1}^{n}\sum _{j=1,j\neq i}^{n}(a_{i}b_{j}-a_{j}b_{i})^{2}\right),\end{aligned}}} which applies to any two sets {a1, a2, ..., an} and {b1, b2, ..., bn} of real or complex numbers (or more generally, elements of a commutative ring). This identity is a generalisation of the Brahmagupta–Fibonacci identity and a special form of the Binet–Cauchy identity. In a more compact vector notation, Lagrange's identity is expressed as: ‖ a ‖ 2 ‖ b ‖ 2 − ( a ⋅ b ) 2 = ∑ 1 ≤ i < j ≤ n ( a i b j − a j b i ) 2 , {\displaystyle \left\|\mathbf {a} \right\|^{2}\left\|\mathbf {b} \right\|^{2}-(\mathbf {a} \cdot \mathbf {b} )^{2}=\sum _{1\leq i<j\leq n}\left(a_{i}b_{j}-a_{j}b_{i}\right)^{2}\,,} where a and b are n-dimensional vectors with components that are real numbers. The extension to complex numbers requires the interpretation of the dot product as an inner product or Hermitian dot product. Explicitly, for complex numbers, Lagrange's identity can be written in the form: ( ∑ k = 1 n | a k | 2 ) ( ∑ k = 1 n | b k | 2 ) − | ∑ k = 1 n a k b k...