Copenhagen is the capital and largest city of Denmark with a population of 794,000. Starting as a Viking fishing village, it became the capitol in the 15th century. Demark was in a union with Sweden and Norway from the 15th to the 16th century. The city saw a renaissance after being subjected to a plague and a massive fire in the 18th century. In the 19th century Horatio Nelson defeated the Dano-Norwegian fleet and bombarded the city. It's current neoclassical architecture of the city dates from rebuilding in the 19th century.
In "The Peanut Reaction", Howard invites Leonard to a special showing of "Blade Runner" (to get him out of the apartment so Penny can set up his birthday party) with 8 extra seconds of footage, but Leonard is intent on beating a longtime rival video game from Copenhagen.
In "The Collaboration Fluctuation", Sheldon is working at a whiteboard trying to prove Amy's hypothesis using neuroscience and quantum mechanics. They could disprove the role of consciousness in the Copenhagen interpretation.
In "The Zazzy Substitution", Shamy is playing their self-invented game Counter Factuals. Leonard tries the game and fails miserably. Asked that in a world where mankind is ruled by a giant, intelligent beaver, what food is no longer consumed? Leonard's answer is cinnamon, but the correct one is cheese Danish. Since the people would build many dams to appease their overlord and flood the city of Copenhagen, devastating the Danish people enough to never invent their namesake pastry.