How to prove this relation between Ramsey Numbers: $R(s,t)≤R(s,t−1)+R(s−1,t)-1$ for $s,t>2$ when $R(s,t-1)$ and $R(s-1,t)$ are even.
Strengthening of a lemma in Ramsey Theory: $R(m, n) \le R(m-1, n) + R(m, n-1)-1$ if both numbers are even