With Trevelyan Micah

The first three novels written (not published) by Poul Anderson were:

The Broken Sword (fantasy);
Vault Of The Ages (juvenile sf);
Star Ways/The Peregrine (a volume of his first future history).

A future history is event-driven, not character-driven. In this, the Psychotechnic, future history, three characters appear twice and that is it as far as character continuity goes:

"Un-Man," about clones of the dead but well remembered Stefan Rostomily, cameos Fourre;

"Marius," written later but set earlier is about Fourre and cameos Stefan;

in Star Ways, Trevelyan Micah of the Stellar Union Coordination Service bids farewell to Braganza Diane before departing on a mission;

in "The Pirate," written many years later but set a few years earlier, Micah again bids farewell to Diane before departing on a mission.

Thus, Fourre, Micah and Diane each appear twice.

Trevelyan refers to the Second Dark Ages which happened between "Brake" and "Gypsy." When he visits Port Nevada, we read one of Anderson's list-descriptions:

"...humans and nonhumans hustled, jostled, chiseled, brawled, clashed, stole, evangelized, grew rich, grew poor, came, went, and were forgotten..."
-Poul Anderson, Starship (New York, 1982), pp. 213-214.

Trevelyan's nonhuman colleague says of the man whom they are investigating:

"'Our information concerning his world line is fragmentary, and zero about its future segment.'" (p. 220)

Of course information about the future is zero! But does Smokesmith imply that his species does sometimes have information about future segments of world lines? The very fact he (?) discusses a man in terms of his world line is suggestive.

"...the mother-of-pearl iridescence on [Smokesmith's] rugose torso was lovely to watch." (p. 221)

(I took "rugose" to mean "red.")

Nerthus, like Tyrfing, is a name that links Anderson's hard sf to his mythological fiction.