# inducedMap(ToricMap) -- make the induced map between total coordinate rings (a.k.a. Cox rings)

## Synopsis

• Function: inducedMap
• Usage:
inducedMap f
• Inputs:
• f, , with a smooth target
• Optional inputs:
• Degree => ..., default value null, unused
• Verify => ..., default value true, unused
• Outputs:
• , between the total homogeneous coordinate rings (aka Cox rings)

## Description

Any morphism of varieties whose target is a smooth normal toric variety is determined by a collection of lines bundles together with a section of each line bundle. This data defines a ring map whose source is the total coordinate ring (a.k.a. Cox ring) of target variety. For more information, see David A. Cox, "The Functor of a Smooth Toric Variety", The Tohoku Mathematical Journal, Second Series, 47 (1995) 251-262, arXiv:alg-geom/9312001v2.

Given a toric map $f : X \to Y$ where $Y$ is smooth, this method returns the induced map from the total coordinate ring $S$ of $Y$ to the total coordinate ring $R$ of $X$. Since $f$ is torus-equivarient, each variable in the polynomial ring $S$ maps to a monomial in $R$.

As a first example, we compute the map on the total coordinate rings induced by the natural inclusion of the affine plane into projective plane.

 i1 : A = affineSpace 2; i2 : P = toricProjectiveSpace 2; i3 : f = map(P, A, 1) o3 = | 1 0 | | 0 1 | o3 : ToricMap P <--- A i4 : (R = ring A, S = ring P); i5 : f' = inducedMap f o5 = map(R,S,{1, x , x }) 0 1 o5 : RingMap R <--- S i6 : f' vars S o6 = | 1 x_0 x_1 | 1 3 o6 : Matrix R <--- R i7 : ideal f == 0 o7 = true i8 : degrees source f' o8 = {{1}, {1}, {1}} o8 : List i9 : degrees target f' o9 = {{}, {}} o9 : List i10 : assert (isWellDefined f and isHomogeneous f')

The second example considers the projection from the third Hirzebruch surface to the projective line.

 i11 : X = hirzebruchSurface 3; i12 : Y = toricProjectiveSpace 1; i13 : g = map(Y, X, matrix {{1, 0}}) o13 = | 1 0 | o13 : ToricMap Y <--- X i14 : (R = ring Y, S = ring X); i15 : g' = inducedMap g o15 = map(S,R,{x , x }) 2 0 o15 : RingMap S <--- R i16 : degrees source g' o16 = {{1}, {1}} o16 : List i17 : degrees target g' o17 = {{1, 0}, {-3, 1}, {1, 0}, {0, 1}} o17 : List i18 : assert (isWellDefined g and isHomogeneous g')

In the third example, we consider a third Veronese embedding of the projective line into projective $3$-space.

 i19 : Z = toricProjectiveSpace 3; i20 : h = map(Z, Y, matrix {{1}, {2}, {3}}) o20 = | 1 | | 2 | | 3 | o20 : ToricMap Z <--- Y i21 : (R = ring Y, S = ring X); i22 : h' = inducedMap h 3 2 2 3 o22 = map(R,QQ[x ..x ],{x , x x , x x , x }) 0 3 0 0 1 0 1 1 o22 : RingMap R <--- QQ[x ..x ] 0 3 i23 : degrees source g' o23 = {{1}, {1}} o23 : List i24 : degrees target g' o24 = {{1, 0}, {-3, 1}, {1, 0}, {0, 1}} o24 : List i25 : ideal h 2 2 o25 = ideal (x - x x , x x - x x , x - x x ) 2 1 3 1 2 0 3 1 0 2 o25 : Ideal of QQ[x ..x ] 0 3 i26 : assert (isWellDefined h and isHomogeneous h' and ideal h == ker h')

To ensure that the induced map is homogeneous, the optional argument DegreeMap is used to record the degree of the monomials in the target ring $R$.

 i27 : code (inducedMap, ToricMap) o27 = -- code for method: inducedMap(ToricMap) /usr/share/Macaulay2/NormalToricVarieties/ToricMaps.m2:395:37-405:73: --source code: inducedMap ToricMap := RingMap => o -> (cacheValue symbol inducedMap) (f -> ( Y := target f; S := ring Y; R := ring source f; m := picardGroup f; -- degree map map(R, S, apply(numgens S, i -> ( exps := entries pullback(f, Y_i); product(numgens R, j -> R_j^(exps#j)))), DegreeMap => (deg -> first entries (matrix{deg} * transpose m))) ) )

## Caveat

This method assumes that the target is smooth. One may verify this by using isSmooth(NormalToricVariety).