However , officially his surname remained his mother 's : van Dis .
Nobody was allowed to know this , and so , for the sake of the outside world , Adriaan took on his father 's surname : Mulder .
Officially Adriaan 's surname remained his mother 's Adriaan   Dis .
Nobody was allowed to know this , and so , for the sake of the outside world , Adriaan took on his father 's surname : Mulder .