Public keys in the form of 04[x,y] can be compressed since the x axis is symmetrical. Hence we only need the x-coordinate with the 02/03 prefix which states if Y is odd or even. I don’t understand the latter, are odd/even on an elliptic curve the same as pos/neg on a Cartesian plain? This makes sense since negative integers can’t be expressed in keys. So would the ‘negative’ Y-coordinate correspond to an ‘odd’ (03 prefix). If so, how are these coordinates differ in odd/even if the x axis is symmetrical?