Consider a complete graph $G$ with vertex $v_1,v_2 , \dots , v_{n+1}$ and for $1 \leq i < j \leq n+1$ color the edge between $v_i , v_j$ with color of $\left \{ i,i+1,\dots , j-1 \right \}$ . Now if $n > R_{1391}$ there is a monochromatic triangle in $G$ , namely $v_x v_y v_z$ for $x<y<z$ . Then these three sets got same color : $A= \left \{x,x+1,\dots , y-1 \right \}$ , $B= \left \{y,y+1,\dots , z-1 \right \}$ , $A \cup B = \left \{x,x+1,\dots , z-1 \right \}$