For clarification of some of the Scandinavian discussions, such as that of the potential Danish origins of Rollo, I went over my sources for Danish kings (for this period, mostly McTurk and Glyn Jones):

Horicus (I) named in Frankish annals 813-54, when he and all of his family but a child were said to have been killed in a successional dispute.

Horicus (II) presumably the one left standing in 854, last named in Frankish sources in 864.

Sigifridus (and Halbdeni (Halfdan)), brothers who in 873 negotiated with Louis the Child. (Halfdan is never called king, and was probably negotiating for Sigifredus.) Relationship to Horicus I or II unknown, but unlikely for chronological reasons, since Horicus II was a child, and the only survivor of his clan in 854.

Heiligo (Helgi) (aft. 891), named by Adam of Bremen, was well liked by everyone, so he had to go, being overthrown by his successor, (and in one source (Taube) said to have gone on to Russia, where as Oleg of Kiev, he was foster father (and father-in-law) of the Rurikid Igor). Again, no known relation to any of the prior kings.

Olaf, of Swedish, origin took over. Named by Adam.

Chnob/Gnupa/Chnuba, named on two runic inscriptions and as son and successor of Olaf by Adam. Negotiated with, and accepted baptism from the Germans in 934.

Sigtrygg/Sigerich, son of Chnuba, named by Adam and on runic inscriptions. He probably died shortly after his father, as the inscriptions in his memory were executed at the behest of his mother. He is not named in Frankish sources.

Hardegon (Harthecnut?) son of Svein, and from the lands of the Northmanii, named in the genealogy of Svein Estrithson as king, but clearly contemporary with above. (It is not at all unlikely that Denmark was not united at this time, with the dynasty of Olaf and of Gorm established in different regions. One source (I don't buy it) claims that he is the same as the Cnut who had coins inscribed at York.

Gorm, son of Harthecnut, a heathen in 935 (note this is only a year after the baptism of Chnuba, so the Germans and French couldn't have been pleased by the reversal). Certainly identical to Gorm the Old.

Todd A. Farmerie