# ApCoCoA-1:BB.ASneighbors

## BB.ASneighbors

Compute a list of across-the-street neighbors.

### Syntax

```BB.ASneighbors(OO:LIST):LIST
```

### Description

Computes the list of across-the-street neighbors in the border of the order ideal OO. The input is a list OO of terms that specify an order ideal. The output is a list of quadruples <formula>[i,j,k,l]</formula> such that <formula>x_k * b_i = x_l * b_j</formula>.

• @param OO A list of terms representing an order ideal.

• @return A list of across-the-street neighbors of OO.

#### Example

```Use Q[x,y,z];
BB.ASneighbors([1,x]);
[[1, 2, 2, 3], [3, 4, 2, 3], [3, 5, 1, 3], [4, 5, 1, 2]]
-------------------------------
```
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