Knutson Family

I’m looking for info on my 3x great grandparents, Jorgen and Asjer Knutson. Jorgen was born about 1825, Norway and died 17 Aug 1884. Supposedly in Minnesota (the Polk County area). Asjer was born 8 Jun 1838, Norway and died 17 Oct 1909 in Hubbard Township. The death record of their daughter, my 2x great grandmother, Oline Berthea Knutson, states that Asjer’s last name is Underberg. Possibly her maiden name? I don’t have either Jorgen’s or Asjer’s birth records, and I don’t know where in Norway they are from. Anything helps, much appreciated.