Quintic point group operation

Suppose we have a quintic curve in the form

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle y^2=px^5+qx^4+rx^3+sx^2+tx+u} .

Through any four points on a quintic curve in this form, an ordinary elliptic curve in the form

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle y^2=ax^3+bx^2+cx+d}

may be fitted, and this elliptic curve must intersect the quintic curve at a fifth point, uniquely determined, up to sign of the y-axis.

The quincunx equations

If P, Q, R, S and T are the five points of intersection between the quintic curve and the fitted elliptic curve, permitting multiplicity, let

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P+Q+R+S+T=O} ,

where O is the additional “point at infinity” considered to lie on the curve and serve as an identity for its additive point group operation.

Curve fittings through one point

Choose a point P and solve Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 4P+T=O} , Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 3P+2S=O} , and $2P+3R=P+4Q=O$ by curve-fitting. Now $T=-4P$, $S=-{\frac {3}{2}}P$, $R=-{\frac {2}{3}}P$, and $Q=-{\frac {1}{4}}P$.

$\{P\}\longrightarrow \left\{-4P,-{\frac {3}{2}}P,-{\frac {2}{3}}P,-{\frac {1}{4}}P\right\}$

Curve fittings through two points

Choosing points P and Q it is possible to solve for $2P+2Q+T=O$ and $P+Q+3R=O$ by curve-fitting: $T=-2(P+Q)$ and $R=-{\frac {1}{3}}(P+Q)$, if scalar division is permitted.

$\{P,Q\}\longrightarrow \left\{-2(P+Q),-{\frac {1}{3}}(P+Q)\right\}$

Goals and objectives

The goal is to reach the point $-P$ from the point P alone, and to reach the point Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P+Q} from the points P and Q if possible, and if so, by the shortest possible path of computation, i.e., using the least number of elliptic curve fittings.