Reduced Temperley-Lieb skein theory

At $A=e^{\pi i/(2r)}$, $r\ge 3$, the projector $f_{r-1}$ exists but its closed trace is zero. Quotient by the skein ideal generated by this projector. The surviving simple colors are $0,\dots ,r-2$, with nonzero quantum dimensions $d_j=(-1{)}^j\sin ((j+1)\pi /r)/\sin (\pi /r)$. This truncation prevents zero-trace channels from interfering with nondegenerate fusion and surgery normalization.

A surviving color denotes $j$ parallel framed strands equipped with the Jones-Wenzl idempotent $f_j$. Its closed trace is its quantum dimension. The semisimplicity of the reduced Temperley-Lieb category identifies these colors with the simple objects; their tensor products decompose according to fusion of Jones-Wenzl colors.

The Jones-Wenzl idempotent recursion decomposes $f_j\otimes 1$ into the orthogonal $f_{j+1}$ and $f_{j-1}$ channels. The Jones-Wenzl partial trace supplies the nonzero scalar normalizing the cap-cup channel. At the highest surviving color the $f_{r-1}$ channel is zero. Induction therefore decomposes every tensor power into the surviving colors. A planar matching between different projected colors has a turnback on the larger side and vanishes; between equal colors only the identity matching remains. Their endomorphism spaces are consequently one-dimensional, and the resulting finite matrix blocks split. Taking direct sums and splitting these idempotents produces a semisimple category, supplying the complete resolutions needed in fusion of Jones-Wenzl colors and the Kirby color handle-slide identity.

In the reduced theory the product of colors a and b decomposes with multiplicity one into colors c satisfying $|a-b|\le c\le \min (a+b,2r-4-a-b)$ and $a+b+c$ even. These rules follow by iterating $1\otimes j=(j-1)\oplus (j+1)$, with color $r-1$ set to zero. The closure traces give $d_a{d}_b={\sum }_c{N}_{ab}^c{d}_c$, and self-duality makes the multiplicities symmetric in all three colors.

In the reduced Jones-Wenzl theory, bend a fixed self-dual color a in the mutually inverse inclusion/projection maps of $c\subset b\otimes a$, obtaining $U_c^b:b\to c\otimes a$ and $V_c^b:c\otimes a\to b$. Cyclic closure trace and straightening cups and caps give $V_c^b{U}_c^b=(d_c/d_b)1_b$. Completeness of the normalized maps for all b then proves the weighted resolution. The dimensions are nonzero in the reduced theory. This local identity gives the precise weight preservation in the Kirby color handle-slide identity.

The full bracket of a zero-framed Hopf link colored i and j is the displayed pairing. Its normalized matrix is the finite sine transform with parity signs. Sine orthogonality gives $H^2={\mathcal{D}}^{2}I$, where ${\mathcal{D}}^2={\sum }_j{d}_j^2=r/(2{\sin }^2(\pi /r))$. Thus the pairing is nondegenerate in the reduced theory.