Question: The Passacaglia for orchestra, Opus 1, is the first published composition of twentieth century Austrian composer Anton Webern, who was mentored by who?

Context paragraphs:
Rondo in C minor (Bruckner): The Rondo in C minor (WAB deest) is a composition for string quartet by the Austrian composer Anton Bruckner.  It was written in 1862 but was not performed publicly until 1984, after the composer's death.  A critical edition was first published in 1985 and the piece was first recorded in 1992 by the Raphael Quartet.

Symphony No. 2 (Schnittke): Russian composer Alfred Schnittke wrote his Symphony No. 2, subtitled "St. Florian" and "Invisible Mass" in 1979.  It is a choral symphony, written for contralto, countertenor, tenor and bass, plus chorus and orchestra.  The symphony was written in homage to 19th century Austrian composer and organist Anton Bruckner, who was closely associated with St. Florian's Priory in the town of Sankt Florian, Upper Austria, and who is buried under the organ there.

Jacques-Louis Monod: Jacques-Louis Monod (born February 25, 1927) is a French-born, naturalized American composer, pianist and conductor of 20th century and contemporary music, particularly in the advancement of the music of Charles Ives, Arnold Schoenberg, Anton Webern and uptown music; and was active primarily in New York City and London during the second half of the twentieth century.

Different Drummer: Different Drummer is a one-act ballet created by Kenneth MacMillan in 1984 for the Royal Ballet.  The music is by Anton Webern (Passacaglia for Orchestra, Op. 1) and Arnold Schoenberg ("Verklärte Nacht").  The story is based on the play "Woyzeck" by Georg Büchner.

Karl Weigl: Karl Ignaz Weigl (6 February 1881 – 11 August 1949) was an Austrian composer.  He was born in Vienna, being the son of a bank official who was also a keen amateur musician.  Alexander Zemlinsky took him as a private pupil in 1896.  Weigl went to school at the Franz-Joseph-Gymnasium and graduated from there in 1899.  After that, he continued his studies at the Vienna Music Academy, where he became a composition pupil of Robert Fuchs, and also enrolled at the University of Vienna, studying musicology under Guido Adler, having Anton Webern as a classmate.  His only opera, "Der Rattenfänger von Hameln", premiered in Vienna in 1932.

Intermezzo in D minor (Bruckner): The Intermezzo in D minor (WAB 113) is an 1879 composition by the Austrian composer Anton Bruckner.  Although it was intended to replace a movement of the String Quintet, that piece was instead performed in its original form; the Intermezzo was not publicly premiered until after the composer's death.

Elena Asachi: Elena Asachi, née Teyber, (b. 30 October 1789, d. 9 May 1877) was a Romanian pianist, singer and composer of Austrian birth.  She was the daughter of Austrian composer Anton Teyber and niece of concertmaster Franz Teyber.

Anton Webern: Anton Friedrich Wilhelm (von) Webern (] ; 3 December 188315 September 1945) was an Austrian composer and conductor.  Along with his mentor Arnold Schoenberg and his colleague Alban Berg, Webern was at the core among those within and more peripheral to the circle of the Second Viennese School, including Ernst Krenek and Theodor W. Adorno.  As an exponent of atonality and twelve-tone technique, Webern exerted influence on contemporaries Luigi Dallapiccola, Křenek, and even Schoenberg himself.  As a tutor, Webern guided and variously influenced Arnold Elston, Frederick Dorian (Friederich Deutsch), Matty Niël, Fré Focke, Karl Amadeus Hartmann, Philipp Herschkowitz, René Leibowitz, Humphrey Searle, Leopold Spinner, and Stefan Wolpe.

Passacaglia for orchestra (Webern): The Passacaglia for orchestra, Opus 1, is the first published composition of twentieth century Austrian composer Anton Webern.  It is based on the 17th century musical form, the passacaglia.

List of compositions by Anton Webern: This is a list of compositions by Anton Webern, the Austrian composer and conductor.

Supporting paragraphs: Passacaglia for orchestra (Webern) Anton Webern
Answer: Arnold Schoenberg