\begin{xy}
\xymatrix {
*\txt{start} \ar[r]
& *++[o][F-]{0} \ar@(r,u)[]^b \ar[r]_a
& *++[o][F-]{1} \ar[r]^b \ar@(r,d)[]_a
& *++[o][F-]{2} \ar[r]^b
\ar `dr_l[l] `_ur[l] _(.2)a[l]
& *++[o][F=]{3}
\ar `ur^l[lll] `^dr[lll]^b [lll]
\ar `dr_l[ll] `_ur[ll] [ll]
}
\end{xy}
\begin{xy}\xymatrix {*\txt{start} \ar[r]& *++[o][F-]{0} \ar@(r,u)[]^b \ar[r]_a& *++[o][F-]{1} \ar[r]^b \ar@(r,d)[]_a& *++[o][F-]{2} \ar[r]^b\ar `dr_l[l] `_ur[l] _(.2)a[l]& *++[o][F=]{3}\ar `ur^l[lll] `^dr[lll]^b [lll]\ar `dr_l[ll] `_ur[ll] [ll]}\end{xy}
\begin{xy}
0;<1em,0em>:
(0,0);p+/v a(18) 3em/**@{.} ?>*@{>},
(0,-1.5);p+/v a(18) 3em/**@{.} ?>*@{<},
(0,-3);p+/v a(18) 3em/**@{.} ?>*@{|},
(0,-4.5);p+/v a(18) 3em/**@{.} ?>*@{(},
(0,-6);p+/v a(18) 3em/**@{.} ?>*@{)},
(5,0);p+/v a(18) 3em/**@{.} ?>*@^{>},
(5,-1.5);p+/v a(18) 3em/**@{.} ?>*@^{<},
(5,-3);p+/v a(18) 3em/**@{.} ?>*@^{|},
(5,-4.5);p+/v a(18) 3em/**@{.} ?>*@^{(},
(5,-6);p+/v a(18) 3em/**@{.} ?>*@^{)},
(5,-7.5);p+/v a(18) 3em/**@{.} ?>*@^{`},
(5,-9);p+/v a(18) 3em/**@{.} ?>*@^{'},
\end{xy}
\begin{xy}0;<1em,0em>:(0,0);p+/v a(18) 3em/**@{.} ?>*@{>},(0,-1.5);p+/v a(18) 3em/**@{.} ?>*@{<},(0,-3);p+/v a(18) 3em/**@{.} ?>*@{|},(0,-4.5);p+/v a(18) 3em/**@{.} ?>*@{(},(0,-6);p+/v a(18) 3em/**@{.} ?>*@{)},(5,0);p+/v a(18) 3em/**@{.} ?>*@^{>},(5,-1.5);p+/v a(18) 3em/**@{.} ?>*@^{<},(5,-3);p+/v a(18) 3em/**@{.} ?>*@^{|},(5,-4.5);p+/v a(18) 3em/**@{.} ?>*@^{(},(5,-6);p+/v a(18) 3em/**@{.} ?>*@^{)},(5,-7.5);p+/v a(18) 3em/**@{.} ?>*@^{`},(5,-9);p+/v a(18) 3em/**@{.} ?>*@^{'},\end{xy}
$$\newdir{ >}{{}*!/-3pt/@{>}}
\begin{xy}
0*+{A}="a", <5pc, 1pc>*+{B}="b"
\ar @{>->} "a";"b" <2pt>
\ar @{ >->} "a";"b" <-2pt>
\end{xy}$$
$$\newdir{ >}{{}*!/-3pt/@{>}}\begin{xy}0*+{A}="a", <5pc, 1pc>*+{B}="b"\ar @{>->} "a";"b" <2pt>\ar @{ >->} "a";"b" <-2pt>\end{xy}$$
\begin{xy}
<4pc,0pc>:(0,0)
*+{base}="base"
\PATH ~={**@{-} ?>*@{>}}
`l (-1,-1)*{A} ^a
` (1,-1) *{B} ^b
`_ul (1,0) *{C} ^c
`ul^l "base" ^d
"base" ^e
\end{xy}
\begin{xy}<4pc,0pc>:(0,0)*+{base}="base"\PATH ~={**@{-} ?>*@{>}}`l (-1,-1)*{A} ^a` (1,-1) *{B} ^b`_ul (1,0) *{C} ^c`ul^l "base" ^d"base" ^e\end{xy}
\begin{xy}
0;<0.8pc,0pc>:
(0,0)="o", "o"*!/rd 1em/{O},
"o"+/l 3pc/="xs";"o"+/r 13pc/="xe" **@{-} ?>*@{>} ?>*!/u 1em/{x},
"o"+/d 3pc/="ys";"o"+/u 8pc/="ye" **@{-} ?>*@{>} ?>*!/r 1em/{y},
(13,10)*{y=f(x)},
(13,-3)*{x_{n+1} = x_n - \frac{f(x_n)}{f'(x_n)}},
(13.5,0)="x0" *!/u 1em/{x_0},
(-3,-4)="A", (15,9)="B", (1.5,5)="C", (10,-2.5)="D",
"A";"B" **\crv{"C"&"D"},
?!{"x0"+/d 3pc/;"x0"+/u 10pc/}="fx0" +/3pc/="L1e" -/12pc/="L1s";"L1e" **\dir{--},
?!{"xs";"xe"}="x1" *!/u 1em/{x_1},
"fx0";"fx0"+/l 20pc/ **@{} ?!{"ys";"ye"}="y0" *!/r 1em/{f(x_0)},
"fx0";"y0" **@{.},
"x0";"fx0" **@{.},
"L1e" *!/l 5em/{y=f(x_0)+f'(x_0)(x-x_0)},
"A";"B" **\crv{~**@{} "C"&"D"},
?!{"x1"+/d 3pc/;"x1"+/u 10pc/}="fx1" +/5pc/="L2e" -/15pc/="L2s";"L2e" **\dir{--},
?!{"xs";"xe"}="x2" *!/u 1em/{x_2},
"fx1";"fx1"+/l 20pc/ **@{} ?!{"ys";"ye"}="y1" *!/r 1em/{f(x_1)},
"fx1";"y1" **@{.},
"x1";"fx1" **@{.},
"L2e" *!/l 5em/{y=f(x_1)+f'(x_1)(x-x_1)},
\end{xy}
\begin{xy}
0;<0.8pc,0pc>:
(0,0)="o", "o"*!/rd 1em/{O},
"o"+/l 3pc/="xs";"o"+/r 13pc/="xe" **@{-} ?>*@{>} ?>*!/u 1em/{x},
"o"+/d 3pc/="ys";"o"+/u 8pc/="ye" **@{-} ?>*@{>} ?>*!/r 1em/{y},
(13,10)*{y=f(x)},
(13,-3)*{x_{n+1} = x_n - \frac{f(x_n)}{f'(x_n)}},
(13.5,0)="x0" *!/u 1em/{x_0},
(-3,-4)="A", (15,9)="B", (1.5,5)="C", (10,-2.5)="D",
"A";"B" **\crv{"C"&"D"},
?!{"x0"+/d 3pc/;"x0"+/u 10pc/}="fx0" +/3pc/="L1e" -/12pc/="L1s";"L1e" **\dir{--},
?!{"xs";"xe"}="x1" *!/u 1em/{x_1},
"fx0";"fx0"+/l 20pc/ **@{} ?!{"ys";"ye"}="y0" *!/r 1em/{f(x_0)},
"fx0";"y0" **@{.},
"x0";"fx0" **@{.},
"L1e" *!/l 5em/{y=f(x_0)+f'(x_0)(x-x_0)},
"A";"B" **\crv{~**@{} "C"&"D"},
?!{"x1"+/d 3pc/;"x1"+/u 10pc/}="fx1" +/5pc/="L2e" -/15pc/="L2s";"L2e" **\dir{--},
?!{"xs";"xe"}="x2" *!/u 1em/{x_2},
"fx1";"fx1"+/l 20pc/ **@{} ?!{"ys";"ye"}="y1" *!/r 1em/{f(x_1)},
"fx1";"y1" **@{.},
"x1";"fx1" **@{.},
"L2e" *!/l 5em/{y=f(x_1)+f'(x_1)(x-x_1)},
\end{xy}
Cú pháp quá phức tạp ! @@
Để hiểu thêm, tham khảo tài liệu của gói XY-PIC.