Chern class of the tangent bundle of a product of projective lines.

The formula is:

c(T(P^1 x P^1 ... x P^1)) = prod_i (1 + alpha + beta + 2*omega_i)

because

c(T(PV)) = \prod_k (1 + w_i + omega)  `mod`  prod_k (w_i + omega) 

and

(1+alpha+omega) * (1+beta+omega) = 1 + alpha + beta + 2*omega 

since the quadratic term is c_2 of a line bundle which is zero

Diagonal embeddings of ordered products of P^1-s

The CSM of the small diagonal in P^1 x ... x P^1

Recursively compute the CSM of the Zariski-open set U^n of distinct ordered points in Q^d = P^1 x ... x P^1. We can compute this by we can subtract all the distinct fat diagonals from the Chern class of Q^d, and the diagonals are just pushforwards of the same thing for smaller d-s.

NOTE: We also have a more explicit formula for the result (which is much faster to compute) and we can compare the two.

Note: Forgetting the alpha/beta part, this should equal to

(1-h1-h2-...-hd)^(d-3)

But, remember that in this formula, h_i^2 = 0 for all i!

Including also alpha and beta we have instead the umbral formula

(q-h1-h2-...-hd)^(d-3)

where we also have to do the umbral substitution q^k -> Q_k, and the polynomials Q_k(alpha,beta) are defined recursively, and are defined for k >= -3.

Simply the pushforward of the CSM of the open stratum along the diagonal map corresponding to the given set partition

We sum over the closure

A formal monomial q^k

The umbral formula for the open stratum of the CSM of distinct ordered point:

(q - u1 - u2 - ... - un)^(n-3)

where u_i^2 = 1. This also works n = 0,1,2,3 For these we have the expansion:

(q - u1 - u2 - u3)^0   =  q^0
(q - u1 - u2     )^-1  =  1/q + u1/q^2 + u2/q^2 + (2*u1*u2)/q^3
(q - u1          )^-2  =  1/q^2 + (2*u1)/q^3
(q               )^-3  =  1/q^3

Given a function specifying what to substitute in the place of q^k, we do the substitution.

It is not hard to prove (by considering the pushforward along the map forgetting one of the points), that the CSM of the locus U^n of the distinct points has the following form (for n>=3):

csm(U^n) = sum_{k=0}^{n-3} \frac{(n-3)!}{k!} (-1)^{n-3-k} \sigma_{n-3-k}(u) Q_k(a,b)

We can already compute all CSM-s recursively, and from that information we can determine these polynomials.

Which then we can compare with the recursive formula for the polynomials itself (which is much faster to evaluate)

The Fibonacci-type recursive formula for the Q_k(a,b) polynomials

Q_{-3} = 1
Q_k    = Q_{k-1} * (1 - (k+1)*(a+b)) - Q_{k-2} * a*b * (k-1)*(k+2)
       = Q_{k-1} * (1 - (k+1)* c_1 ) - Q_{k-2} * c_2 * (k-1)*(k+2)

We provide both the Chern root and the Chern class version in a uniform way for convenience.

The formula for the CSM of the set of distinct ordered points using the formula for the Q_k(a,b) polynomials above

Just the pushforward of the previous along Delta_mu