Decimal point position in result is wrong for non-integral numbers.
Sign is wrong for negative numbers.
==== Here is the minimized test demonstrating the bug ====
class java_lang_Bignum_pow {
  public static void main(String args[]) {
    Bignum b1=new Bignum("2.0");
    Bignum b2=new Bignum("-2");
    System.out.println(b1.pow(2)); //should print "4.0"
    System.out.println(b2.pow(2)); //should print "4"
  }
}
==== Here is the test output ====
40.0
-4