For the secp256k1 curve, by coordinate values
(((x_1)^4−56(x_1))/(4(x_1)^3+28))=(x_2) from this we get
x_1 = x_2 - 1/2 sqrt(4 x_2^2 - (4 2^(1/3) 7^(2/3) x_2)/(-x_2^3 + sqrt(x_2^6 + 14 x_2^3 + 49) + 7)^(1/3) + 2 2^(2/3) 7^(1/3) (-x_2^3 + sqrt(x_2^6 + 14 x_2^3 + 49) + 7)^(1/3)) - 1/2 sqrt(8 x_2^2 + (4 2^(1/3) 7^(2/3) x_2)/(-x_2^3 + sqrt(x_2^6 + 14 x_2^3 + 49) + 7)^(1/3) - 2 2^(2/3) 7^(1/3) (-x_2^3 + sqrt(x_2^6 + 14 x_2^3 + 49) + 7)^(1/3) - (64 x_2^3 + 448)/(4 sqrt(4 x_2^2 - (4 2^(1/3) 7^(2/3) x_2)/(-x_2^3 + sqrt(x_2^6 + 14 x_2^3 + 49) + 7)^(1/3) + 2 2^(2/3) 7^(1/3) (-x_2^3 + sqrt(x_2^6 + 14 x_2^3 + 49) + 7)^(1/3))))
((y_1)^4+126*(y_1)^2-1323)/(8(y_1)^3)=y_2 from this we get
y_1 = 2 y_2 - sqrt(4 y_2^2 + 3 7^(2/3) (7 - y_2^2)^(1/3) - 21) - 1/2 sqrt(32 y_2^2 - 12 7^(2/3) (7 - y_2^2)^(1/3) - (512 y_2^3 - 4032 y_2)/(8 sqrt(4 y_2^2 + 3 7^(2/3) (7 - y_2^2)^(1/3) - 21)) - 168)
