Algebraic coefficients of fractional parentage for three particles in the oscillator Jacobi basis
We present a closed-form construction of the orbital coefficients of fractional parentage (CFP) of three identical particles in the translationally invariant harmonic oscillator (HO) Jacobi basis. These are usually obtained by numerically diagonalizing a permutation operator in every energy-angular-momentum block. We show this diagonalization is unnecessary. Dressing the class operator $P_{13} + P_{23}$ with the single unitary $W = H^1 Φ$ -- a fixed complexification followed by the HO bracket with mass parameter $d=1$ -- turns it into the label swap times an integer. Every CFP is then the single ket $|K_{i,s}\rangle = 2Q^s W^\dagger|i\rangle$ -- a 2-scaled, sign-decorated column of the $d=1$ bracket restricted to one $e_2$-parity of the labels -- whose norm, $1 + s\langle i | J | i \rangle$, is itself a single matrix element. The same $W$ carries label parity into swap symmetry, $W Q^s W^\dagger = R^s$, making the bookkeeping collapse. The eigenvalues are exact integers $\pm 2$ and $\pm 1$ labeling the $S(3)$ symmetry classes. The construction is uniform in $E$, $L$ and symmetry class, reducing CFP-specific work from $O(n^3)$ to $O(n^2)$ in block dimension. An eigensolver remains only inside the bracket engine supplying $H^1$, where all residual numerical error originates. We implement this on an SU(3)-basis bracket engine, checking all 1301 blocks up to $E=50$ (912 366 CFPs, largest block $2907 \times 2907$): the $S(3)$ class assignment is exact everywhere. The residual numerical error (entirely the bracket engine's) stays below $10^{-8}$ for $E \leq 35$, restoring to $\leq 10^{-22}$ via a quadruple-precision build beyond that. We further validate against independently generated antisymmetrization data of a production three-nucleon code, including the $^3$He ground-state energy.