English
Degen’s eight-square identity expresses the product of sums of eight squares as a sum of eight squares with a prescribed sign pattern.
Русский
Идентичность восьми квадратов Дегена выражает произведение сумм восьми квадратов как сумму восьми квадратов с заданной схемой знаков.
LaTeX
$$Equality of eight-square identity as stated in the theorem.$$
Lean4
/-- Degen's eight squares identity, see <https://en.wikipedia.org/wiki/Degen%27s_eight-square_identity>.
This sign choice here corresponds to the signs obtained by multiplying two octonions.
-/
theorem sum_eight_sq_mul_sum_eight_sq :
(x₁ ^ 2 + x₂ ^ 2 + x₃ ^ 2 + x₄ ^ 2 + x₅ ^ 2 + x₆ ^ 2 + x₇ ^ 2 + x₈ ^ 2) *
(y₁ ^ 2 + y₂ ^ 2 + y₃ ^ 2 + y₄ ^ 2 + y₅ ^ 2 + y₆ ^ 2 + y₇ ^ 2 + y₈ ^ 2) =
(x₁ * y₁ - x₂ * y₂ - x₃ * y₃ - x₄ * y₄ - x₅ * y₅ - x₆ * y₆ - x₇ * y₇ - x₈ * y₈) ^ 2 +
(x₁ * y₂ + x₂ * y₁ + x₃ * y₄ - x₄ * y₃ + x₅ * y₆ - x₆ * y₅ - x₇ * y₈ + x₈ * y₇) ^ 2 +
(x₁ * y₃ - x₂ * y₄ + x₃ * y₁ + x₄ * y₂ + x₅ * y₇ + x₆ * y₈ - x₇ * y₅ - x₈ * y₆) ^ 2 +
(x₁ * y₄ + x₂ * y₃ - x₃ * y₂ + x₄ * y₁ + x₅ * y₈ - x₆ * y₇ + x₇ * y₆ - x₈ * y₅) ^ 2 +
(x₁ * y₅ - x₂ * y₆ - x₃ * y₇ - x₄ * y₈ + x₅ * y₁ + x₆ * y₂ + x₇ * y₃ + x₈ * y₄) ^ 2 +
(x₁ * y₆ + x₂ * y₅ - x₃ * y₈ + x₄ * y₇ - x₅ * y₂ + x₆ * y₁ - x₇ * y₄ + x₈ * y₃) ^ 2 +
(x₁ * y₇ + x₂ * y₈ + x₃ * y₅ - x₄ * y₆ - x₅ * y₃ + x₆ * y₄ + x₇ * y₁ - x₈ * y₂) ^ 2 +
(x₁ * y₈ - x₂ * y₇ + x₃ * y₆ + x₄ * y₅ - x₅ * y₄ - x₆ * y₃ + x₇ * y₂ + x₈ * y₁) ^ 2 :=
by ring
