f$1($t$)=$n$=0$$(1)$nu$($t$$n$)$f$\_1($t$)=\backslash$sum$\_\{$n$=0\}^\backslash$infty $(-1)^$n u$($t $-$ n$)$f$1($t$)=$n$=0$$(1)$nu$($t$$n$)$

f$2($t$)=$n$=0$u$($t$$n$)$f$\_2($t$)=\backslash$sum$\_\{$n$=0\}^\backslash$infty u$($t $-$ n$)$f$2($t$)=$n$=0$$$u$($t$$n$)$

f$3($t$)=$t$$t$$f$\_3($t$)=$ t $-\backslash$lfloor t $\backslash$rfloorf$3($t$)=$t$$t$$