can anyone enlighten me as to what the inverted 6 symbol means? its the number 6 inverted left to right. i came across it in my calculus notes under partial derivatives under functions of two variables.

the formula goes

fx = (inverted 6)f/(inverted6)x = (inverted 6)z/(inverted 6)x

fy = (inverted 6)f/(inverted6)y = (inverted 6)z/(inverted 6)y

they are defined by the limits:

(inverted 6)f/(inverted 6)x = lim (as h approaches 0) [f(x+hy)-f(xy)] /h