Van der Waerden number $W(2,4)$

The smallest number $n$ such that if the integers $1$ to $n$ are colored with $2$ colors, there must be a monochromatic arithmetic progression of length $4$.

Updates

• 1970 Lower bound: $35$
  Chvátal, V. (1970). Some unknown van der Waerden numbers. Combinatorial structures and their applications, 31-33.
  [via Van der Waerden number - Wikipedia]
• 1970 Upper bound: $35$
  Chvátal, V. (1970). Some unknown van der Waerden numbers. Combinatorial structures and their applications, 31-33.
  [via Van der Waerden number - Wikipedia]