Product of 4 Consecutive Integers is One Less than Square/Lemma
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Lemma for Product of 4 Consecutive Integers is One Less than Square
Let $a$, $b$, $c$ and $d$ be consecutive integers.
Let us wish to prove that the product of $a$, $b$, $c$ and $d$ is one less than a square.
Then it is sufficient to consider $a$, $b$, $c$ and $d$ all strictly positive.
Proof
If $0 \in \set {a, b, c, d}$ then $a b c d = 0$ which is $1$ less than $1^2$.
If $a$, $b$, $c$ and $d$ are all negative, then:
- $a b c d = \paren {-a} \paren {-b} \paren {-c} \paren {-d}$
Hence it is sufficient to consider $a$, $b$, $c$ and $d$ all strictly positive.
$\blacksquare$