In an age of specialists, William Bolcom is a throwback, a general practitioner of the music profession. He is equally at home with the popular songs of the 19th century and with the abstractions of the contemporary concert idiom. He composes and he teaches. He performs ragtime on the piano, records and accompanies. He lectures learnedly, and he is a sitdown comic (he tells jokes from the piano stool).

Last night at the Library of Congress, he and violinist Sergiu Luca teamed up for the premiere performance of his Second Sonata.

It is a piece that comfortably and unashamedly assimilates Bolcom's catholic tastes. Without pretension, its four movements manage to combine elements of blues, baroque ostinato patterns, Brucknerian sonorities and Bartokian vitality into a context that is unified by Bolcom's own musical sense of proportion. The structure is simple and clearly outlined, and the whole thing is handed with an underlying sense of humor.

Luca, whose own way with music is warmly romantic and human, rather than ethereal, collaborated sympathetically with Bolcom. The piece was given a very friendly reception.

Even if Luca were not such a compelling performer, he probably would be remarked for the company he keeps, because he certainly does have a knack for picking fine pianists to perform with.

Several years ago he played here with Emanuel Ax, and last night his partner for the rest of the program was Anne Epperson, a marvelously musical and rhythmically exciting pianist.

Together they began the evening with a board and decisive reading of Stravinsky's "Suite Italienne" that was stylish without being precious.

Two Schubert pieces completed the program: the young, simple Sonatina in D Major, which Luca played with straight forward honesty, and the "Rondo Brillant," a virtuoso conceit with which Luca had a splendid time.