# ApCoCoA-1:NCo.Intersection

From ApCoCoAWiki

## NCo.Intersection

Intersection of two finitely generated two-sided ideals in a free monoid ring.

### Description

*Propostion (Intersection of Two Ideals):* Let `G_I` and `G_J` be two sets of non-zero polynomials in the free nomoid ring `K<x_1,...,x_n>`, and let `I` and `J` be two ideals generated by `G_I` and `G_J`, respectively. We choose a new indeterminate `y`, and form the free monoid ring `K<y,x_1,...,x_n>`. Furthermore, let `N` be the ideal generated by the union of `{yf: f in G_I}` and `{(1-y)g: g in G_J}`, and let `C` be the ideal generated by the set `{yx_1-x_1y,...,yx_n-x_ny}` of commutators. Then we have the intersection of `I` and `J` is equal to the intersection of `N+C` and `K<x_1,...,x_n>`.

#### Example

-- Let I be the ideal generated by G_I={xy+z,yz+x}, and J be the ideal generated by G_J={yz+x, zx+y}. -- We compute the intersection of I and J as follows. NCo.SetX("txyz"); -- Let t be an new indeterminate NCo.SetOrdering("ELIM"); -- Choose an elimination word ordering for t F1 := [[1,"xy"], [1,"z"]]; -- xy+z F2 := [[1,"yz"], [1,"x"]]; -- yz+x G1 := [[1,"yz"], [1,"x"]]; -- yz+x G2 := [[1,"zx"], [1,"y"]]; -- zx+y N:=[NCo.Multiply([[1,"t"]],F1), NCo.Multiply([[1,"t"]],F2)]; -- t*F1, t*F2 N:=Concat(N,[NCo.Multiply([[1,""],[-1,"t"]],G1), NCo.Multiply([[1,""],[-1,"t"]],G2)]); -- (1-t)*G1, (1-t)*G2 C:=[[[1,"tx"],[-1,"xt"]],[[1,"ty"],[-1,"yt"]],[[1,"tz"],[-1,"zt"]]]; -- set of commutators G:=Concat(N,C); Gb:=NCo.GB(G,20,50,1); -- Done. ------------------------------- The following information printed by the ApCoCoAServer shows that Gb it is a partial Groebner basis. the number of unselected generators: 0 the number of unselected ObstructionMs: 70 the procedure is interrupted by loop bound! the total number of ObstructionMs: 298 the number of selected ObstructionMs: 43 the number of ObstructionMs detected by Rule 1: 145 the number of ObstructionMs detected by Rule 2: 0 the number of ObstructionMs detected by Rule 3: 40 the number of redundant generators: 6 It is a partial Groebner basis.