fix tests

Math/Matrix/Matrix.php

@@ -884,10 +884,16 @@ class Matrix implements \ArrayAccess, \Iterator
             }
 
             return $matrix;
-        } else {
-            // @todo: implement
-            throw new \Exception('Not yet implemented');
         }
+
+        $eig = new EigenvalueDecomposition($this);
+
+        $d = $eig->getD();
+        for ($i = 0; $d->getM(); ++$i) {
+            $d->matrix[$i][$i] = \pow($d->matrix[$i][$i], $exponent);
+        }
+
+        return $eig->getV()->mult($d)->mult($eig->getV()->inverse());
     }
 
     /**

tests/Math/Matrix/MatrixTest.php

@@ -646,9 +646,25 @@ final class MatrixTest extends \PHPUnit\Framework\TestCase
                 [80.25, 96.75, 113.25],
                 [120.75, 146.25, 171.75],
             ],
-            $m->pow(2.5)->toArray(),
+            $m->pow(2)->toArray(),
             0.1
         );
+
+        $m = Matrix::fromArray([
+            [1, 1, 1],
+            [1, 2, 3],
+            [1, 3, 6],
+        ]);
+
+        self::assertEqualsWithDelta(
+            [
+                [0.8901, 0.5882, 0.3684],
+                [0.5882, 1.2035, 1.3799],
+                [0.3684, 1.3799, 3.1167],
+            ],
+            $m->pow(2 / 3)->toArray(),
+            0.01
+        );
     }
 
     public function testExp() : void