With fierce energy, violinist Robert Mann and pianist Emanuel Ax last night initiated their three-concert Beethoven sonata series at the Library of Congress. It was, as the overflow audience well knew, a historic occasion. The entire cycle of 10 sonatas has been performed at the Library only twice before -- in 1939 by Serkin and Busch and in 1944 by Arrau and Szigeti. If this first concert is indicative, Mann and Ax will prove honorable additions to the list.

Looking very much like father and son, they brought an exceptional unity of concept to the four sonatas on this first program.Both showed a strong sense of structure and an ability to use subtle shifts in tempo to underline form. They took particular pleasure in exploiting the drama of sharp thematic contrasts and sudden dynamic changes, which are favorite Beethoven devices.

Throughout the evening both played with a driving intensity that squeezed every ounce of excitement out of the music. Their control in the fourth sonata was masterful, ranging from inner fury in the outer movements to tender whimsicality in the middle movement. They caught the bristling force of the first sonata and the surging passion of the seventh. Only the fifth suffered from being more muscular than lyrical.

The cycle continues at the Library tonight at 8 and Sunday afternoon at 5.