Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_20640 ( "Danish name" text, "English name" text, "Seat of administration" text, "Largest city" text, "Population (January 1, 2008)" real, "Area (km\u00b2)" real, "Pop. density (per km\u00b2)" text )

### Question ###
In the region where Roskilde is the largest city, what is the seat of administration?

### Accurate SQL ###

SELECT "Seat of administration" FROM table_20640 WHERE "Largest city" = 'Roskilde'