Pushforward fails for the cuspidal cubic immersion

[mahrud]

I don't understand what is going wrong here:

needsPackage "NormalToricVarieties"

P1 = toricProjectiveSpace 1
P2 = toricProjectiveSpace 2
f = inducedMap map(P2, P1, matrix{{-2},{1}})
S = ring P1

pushForward_f S^{2}

This gives the error:

stdio:6:12:(3): error: not implemented yet: unexpected degree map of ring map

Which comes from here:

M2/M2/Macaulay2/m2/pushforward.m2

Lines 147 to 151 in ec65028

	-- let's at least check it splits f's degree map:
	for i from 0 to deglen-1 do (
	e := for j from 0 to deglen-1 list if i === j then 1 else 0;
	if mapbackdeg f.cache.DegreeMap e =!= e
	then error "not implemented yet: unexpected degree map of ring map");

What is this check for?

Note that the map is sending the variables of P^2 to {x_0*x_1^2, x_0^3, x_1^3}, all degree 3, and both source and target are standard graded.

@mikestillman @ggsmith any ideas?

[pzinn]

what happens when you remove the check?

[mahrud]

Gives an inhomogeneous answer.