Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
According to Van Veen (the narrator and main character in VN's novel Ada, 1969), all the hundred floramors (palatial brothels built all over the world by David van Veen, a wealthy architect of Flemish extraction, in memory of his grandson Eric) opened simultaneously on September 20, 1875:
Describing Villa Venus (Eric Veen's floramors), Van Veen (the narrator and main character in VN's novel Ada, 1969) mentions Eric Veen's essay 'Villa Venus: an Organized Dream’ and says that he owns a photostat of Eric's calligraph:
Before showing to Van her larvarium, Ada (the title character in VN's novel Ada, 1969) tells him that old dogs do not go to sleep, quelle idée, they swoon, it is a little syncope:
‘And now,’ she said, and stopped, staring at him.
‘Yes?’ he said, ‘and now?’
In VN’s novel Priglashenie na kazn’ (“Invitation to a Beheading,” 1935) Cincinnatus would discover on the wall of his prison cell a diminutive profile with a large mouselike ear:
In VN’s novel Priglashenie na kazn’ (“Invitation to a Beheading,” 1935) zavodnye dvukhmestnye chasiki (spring-powered, two-seat 'clocklets,' clockwork cycles) are mentioned several times:
Before the family dinner in "Ardis the Second" Van Veen (the narrator and main character in VN's novel Ada, 1969) says that old storytelling devices may be parodied only by very great and inhuman artists:
‘Our great Coppée,’ said Van, ‘is awful, of course, yet he has one very fetching little piece which Ada de Grandfief here has twisted into English several times, more or less successfully.’
‘Oh, Van!’ interjected Ada with unusual archness, and scooped up a handful of salted almonds.
On Demonia (aka Antiterra, Earth's twin planet on which VN's novel Ada, 1969, is set) Pasternak's novel Doctor Zhivago (1957) is known as Les Amours du Docteur Mertvago, a mystical romance by a pastor, Mertvago Forever and Klara Mertvago:
In VN's novel Ada (1969) Mlle Larivière (Lucette's governess) writes fiction under the penname Guillaume de Monparnasse:
‘Well, that bit about spinsters is rot,’ said Van, ‘we’ll pull it off somehow, we’ll become more and more distant relations in artistically forged papers and finally dwindle to mere namesakes, or at the worst we shall live quietly, you as my housekeeper, I as your epileptic, and then, as in your Chekhov, "we shall see the whole sky swarm with diamonds."’
In VN's novel Ada (1969) Sig Leymanksi (the main character in Van Veen's novel Letters from Terra) curtails his name and becomes Professor Leyman:
Describing his novel Letters from Terra, Van Veen (the narrator and main character in VN's novel Ada, 1969) mentions the poet Max Mispel and his article ‘The Weed Exiles the Flower:’