Also known as the Tee-to-pi or Y-Delta transform, the star-triangle transform allow to transform a star shaped tripolar electrical network to a triangle shaped network with he same behavior and backwards. Suppose we start from the following star/T/Y (the two bottom nodes are in fact only one same node, as they are supraconductively connected) :
```     ------   ------
0----| A  |---|  B |----0
------ | ------
|
-
| |
|C|
| |
-
|
0-----------------------0
```
The triangle/Pi/Delta we want to transform to is the following :
```          ------
0---------| c  |--------0
|    ------    |
|              |
-              -
| |            | |
|b|            |a|
| |            | |
-              -
|              |
0-----------------------0
```
The transformation formulas are the as follows :
```a=(A.B+B.C+A.C)/A
b=(A.B+B.C+A.C)/B
c=(A.B+B.C+A.C)/C
```
And backwards :
```A=(b.c)/(a+b+c)
B=(a.c)/(a+b+c)
C=(a.b)/(a+b+c)
```
This is esily proved by calculating the impedance between each node. This gives a set of three equations that can be added / substracted to isolate any of the variables.