# ApCoCoA-1:Num.SubBBABM

## Num.SubBBABM

Computes a border basis of an almost vanishing sub-ideal for a set of points and an ideal using the Num.BBABM algorithm.

### Syntax

```Num.SubBBABM(Points:MAT, Epsilon:RAT, Basis:LIST):Object
Num.SubBBABM(Points:MAT, Epsilon:RAT, Basis:LIST, Delta:RAT, NormalizeType:INT):Object
```

### Description

Please note: The function(s) explained on this page is/are using the ApCoCoAServer. You will have to start the ApCoCoAServer in order to use it/them.

This command computes a border basis of an almost vanishing sub-ideal for a set of points and an ideal using the Num.BBABM algorithm.

The current ring has to be a ring over the rational numbers with a standard-degree

compatible term-ordering. Each row in the matrix Points represents one point, so the number of columns must equal the

number of indeterminates in the current ring.

• @param Points The points for which a border basis is computed.

• @param Epsilon A positive rational number describing the maximal admissible least squares error for a polynomial. (Bigger values for Epsilon lead to bigger errors of the polynomials evaluated at the point set). Epsilon should be in the interval (0,1). As a rule of thumb, Epsilon is the expected percentage of error on the input points.

• @param Basis A set of polynomials in the current ring. This basis defines the ideal in which we compute the basis of the approximate vanishing ideal.

• @return A list of two results. First the border basis as a list of polynomials, second the vector space basis of P/I as a list of terms.

The following parameters are optional:

• @param Delta A positive rational number which describes the computing precision. In different steps, it is crucial, if a value is 0 or not. The algorithm assumes every value in [-Delta, Delta] to be 0. The default value for Delta is 0.00000000001.

• @param NormalizeType A integer of the set {1,2,3,4}. The default value is 2. This parameter describes, if and where required the input points are normalized. If NormalizeType equals 1, each coordinate of a point is divided by the maximal absolute value of all coordinates of this point. This ensures that all coordinates of the points are within [-1,1]. With NormalizeType=2 no normalization is done at all. NormalizeType=3 shifts each coordinate to [-1,1], i.e. the minimal coordinate of a point is mapped to -1 and the maximal coordinate to 1, which describes a unique affine mapping. The last option is NormalizeType=4. In this case, each point is normalized by its euclidean norm. Although NormalizeType=3 is in most cases a better choice, the default value is due to backward compatibility 1.

#### Example

```Use P::=QQ[x,y,z];

Points := Mat([[2/3,0,0],[0,1,0],[0,0,1/3]]);
R:=Num.SubBBABM(Points, 0.1, [x]);
Dec(R[1],2);
R[2];

-- CoCoAServer: computing Cpu Time = 0
-------------------------------
[<quotes>1 xz </quotes>, <quotes>1 xy </quotes>, <quotes>1 x^2 -0.66 x </quotes>]
-------------------------------
[x]
-------------------------------
```