 Four of Stockhausen 's children became professional musicians ( Kurtz 1992 , 202 ) , and he composed some of his works specifically for them . A large number of pieces for the trumpet — from Sirius ( 1975 – 77 ) to the trumpet version of In Freundschaft ( 1997 ) — were composed for and premièred by his son Markus ( Kurtz 1992 , 208 ; M. Stockhausen 1998 , 13 – 16 ; Tannenbaum 1987 , 61 ) . Markus , at the age of 4 years , had performed the part of The Child in the Cologne première of Originale , alternating performances with his sister Christel ( Maconie 2005 , 220 ) . Klavierstück XII and Klavierstück XIII ( and their versions as scenes from the operas Donnerstag aus Licht and Samstag aus Licht ) were written for his daughter Majella , and were first performed by her at the ages of 16 and 20 , respectively ( Maconie 2005 , 430 , 443 ; Stockhausen Texte , 5 : 190 , 255 , 274 ; Stockhausen Texte , 6 : 64 , 373 ) . The saxophone duet in the second act of Donnerstag aus Licht , and a number of synthesizer parts in the Licht operas , including Klavierstück XV ( " Synthi @-@ Fou " ) from Dienstag , were composed for his son Simon ( Kurtz 1992 , 222 ; Maconie 2005 , 480 , 489 ; Stockhausen Texte , 5 : 186 , 529 ) , who also assisted his father in the production of the electronic music from Freitag aus Licht . His daughter Christel is a flautist who performed and gave a course on interpretation of Tierkreis in 1977 ( Stockhausen Texte , 5 : 105 ) , later published as an article ( C. Stockhausen 1978 ) . 
