An Introduction to Sage

Marco Varisco

Algebra/Topology Seminar, February 7, 2013

Sage as a (smart) calculator
A simple algorithm
Some calculus and pretty pictures
Solving equations
Playing with finite groups
Minimal free resolutions of monomial ideals

Sage as a (smart) calculator

sage: 2^16 65536 sage: 2^160 1461501637330902918203684832716283019655932542976 sage: sin(pi/3) 1/2*sqrt(3) sage: numerical_approx(sin(pi/3)); n(sin(pi/3)); sin(pi/3).n() 0.866025403784439 0.866025403784439 0.866025403784439 sage: n(sin(pi/3), digits=50) 0.86602540378443864676372317075293618347140262690519 sage: sin(pi/3).n(digits=3) 0.866 sage: a = 12222 sage: a 12222 sage: factor(a) 2 * 3^2 * 7 * 97 sage: a.factor() 2 * 3^2 * 7 * 97 sage: a.prime_divisors() [2, 3, 7, 97] sage: pd = _ sage: pd [2, 3, 7, 97] sage: pd[0] 2 sage: pd.append('text') sage: pd [2, 3, 7, 97, 'text'] sage: a % 55 12 sage: a.inverse_mod(55) 23 sage: a.inverse_mod(56) Traceback (most recent call last): ... ZeroDivisionError: Inverse does not exist. sage: A = mod(a, 55) sage: type(a); type(A) sage: A^-1 23 sage: a^-1 1/12222 sage: A.multiplicative_order() 4

A simple algorithm

sage: def EA(a, b): ... while b!=0: ... r = a%b ... a = b ... b = r ... return a sage: EA(12222, 55) 1 sage: EA(12222, 550) 2 sage: def EA(a, b, show=False): ... while b!=0: ... r = a%b ... if show: print (a,b,r) ... a = b; b = r ... if a<0: a = -a ... return a sage: EA(12222, 550, show=True) (12222, 550, 122) (550, 122, 62) (122, 62, 60) (62, 60, 2) (60, 2, 0) 2 sage: EA(12222, 550) == gcd(12222, 550) True sage: xgcd(12222, 550) (2, -9, 200) sage: d, s, t = xgcd(12222, 550) sage: d == s*12222 + t*550 True

Some calculus and pretty pictures

sage: f = atan(sqrt(x)) sage: f arctan(sqrt(x)) sage: type(f) sage: If = f.integrate(x) sage: If x*arctan(sqrt(x)) - sqrt(x) + arctan(sqrt(x)) sage: show(If) $\newcommand{\Bold}[1]{\mathbf{#1}}x \arctan\left(\sqrt{x}\right) - \sqrt{x} + \arctan\left(\sqrt{x}\right)$ sage: latex(If) x \arctan\left(\sqrt{x}\right) - \sqrt{x} + \arctan\left(\sqrt{x}\right)

At this point you should check the Typeset box at the top of this worksheet.

sage: DIf = If.differentiate(x) sage: f - DIf $\newcommand{\Bold}[1]{\mathbf{#1}}-\frac{\sqrt{x}}{2 \, {\left(x + 1\right)}} + \frac{1}{2 \, \sqrt{x}} - \frac{1}{2 \, {\left(x + 1\right)} \sqrt{x}}$ sage: (f - DIf).simplify_full() $\newcommand{\Bold}[1]{\mathbf{#1}}0$ sage: f = x*cos(x)^2 sage: f $\newcommand{\Bold}[1]{\mathbf{#1}}x \cos\left(x\right)^{2}$ sage: plot(f) sage: p = plot(f, xmin=-1, xmax=3.5, ymin=-0.5, ymax=3.5, aspect_ratio=.5, gridlines=True, color='purple', thickness=10, alpha=0.2, figsize=6) sage: p

sage: p.save('plot.pdf') sage: plotf = plot(f, (x,-1,3.5), color='purple', thickness=3) sage: origin = point((0,0), color='orange', alpha=.7, size=150) sage: label = 'MacLaurin polynomial of $%s$ of degree'%latex(f) sage: @interact sage: def foo(j=slider(0, 20, 1, default=3, label=label)): ... Tjf = f.taylor(x, 0, j) ... plotTjf = plot(Tjf, (x,-1,3.5), color='green', thickness=1.5, fill=f) ... html('$%s$'%latex(Tjf)) ... show(plotf + plotTjf + origin, ymin=-0.5, ymax=3.5, figsize=[7,3])

MacLaurin polynomial of $x \cos\left(x\right)^{2}$ of degree

 $-x^{3} + x$

sage: frames = [] sage: for j in range(-1, 21, 2): ... Tjf = f.taylor(x, 0, j) ... plotTjf = plot(Tjf, (x,-1,3.5), color='green', thickness=1.5, fill=f) ... t = text('$%s$'%latex(Tjf), (3,-0.8), color='black', horizontal_alignment='right') ... frames.append(t + plotf + plotTjf + origin) ... sage: a = animate(frames, ymin=-0.5, ymax=3.5, figsize=[7,3]) sage: a.show(delay=40, iterations=4) sage: var('x, y, z') sage: f = x^2 + y^2 + z^2 + cos(4*x) + cos(4*y) + cos(4*z) sage: c = 0.2 sage: implicit_plot3d(f==c, (x, -1.2, 1.2), (y, -1.2, 1.2), (z, -1.2, 1.2)) sage: implicit_plot3d(f==c, (x, -1.2, 1.2), (y, -1.2, 1.2), (z, -1.2, 1.2), opacity=2/3) + dodecahedron((0,0,0), 1/2, color="purple", opacity=2/3)

Solving equations

sage: solve(x^3 + 6*x == 20, x) $\newcommand{\Bold}[1]{\mathbf{#1}}\left[x = \left(-3 i - 1\right), x = \left(3 i - 1\right), x = 2\right]$ sage: solve(x^4 + 6*x == 20, x)[0] $\newcommand{\Bold}[1]{\mathbf{#1}}x = -\frac{1}{2} \, \sqrt{\frac{3 \, {\left(\frac{2}{9} \, \sqrt{3} \sqrt{130187} + 18\right)}^{\left(\frac{2}{3}\right)} - 80}{{\left(\frac{2}{9} \, \sqrt{3} \sqrt{130187} + 18\right)}^{\frac{1}{3}}}} \sqrt{\frac{1}{3}} - \frac{1}{2} \, \sqrt{-{\left(\frac{2}{9} \, \sqrt{3} \sqrt{130187} + 18\right)}^{\left(\frac{1}{3}\right)} + \frac{36 \, \sqrt{\frac{1}{3}}}{\sqrt{\frac{3 \, {\left(\frac{2}{9} \, \sqrt{3} \sqrt{130187} + 18\right)}^{\left(\frac{2}{3}\right)} - 80}{{\left(\frac{2}{9} \, \sqrt{3} \sqrt{130187} + 18\right)}^{\frac{1}{3}}}}} + \frac{80}{3 \, {\left(\frac{2}{9} \, \sqrt{3} \sqrt{130187} + 18\right)}^{\left(\frac{1}{3}\right)}}}$ sage: solve(x^5 + 6*x == 20, x) $\newcommand{\Bold}[1]{\mathbf{#1}}\left[0 = x^{5} + 6 \, x - 20\right]$ sage: find_root(x^5 + 6*x == 20, 0, 1) Traceback (most recent call last): ... RuntimeError: f appears to have no zero on the interval sage: find_root(x^5 + 6*x == 20, 0, 2) $\newcommand{\Bold}[1]{\mathbf{#1}}1.59778267898$ sage: var('x, y, z') sage: solve([x + 3*y - 2*z == 5, 3*x + 5*y + 6*z == 7], x, y, z) $\newcommand{\Bold}[1]{\mathbf{#1}}\left[\left[x = -7 \, r_{1} - 1, y = 3 \, r_{1} + 2, z = r_{1}\right]\right]$ sage: A = matrix([[1, 3, -2], [3, 5, 6]]) sage: v = vector([5, 7]) sage: A.solve_right(v) $\newcommand{\Bold}[1]{\mathbf{#1}}\left(-1,\,2,\,0\right)$ sage: A.right_kernel() $\newcommand{\Bold}[1]{\mathbf{#1}}\mathrm{RowSpan}_{\Bold{Z}}\left(\begin{array}{rrr} 7 & -3 & -1 \end{array}\right)$ sage: Av = A.augment(v) sage: Av.echelon_form() $\newcommand{\Bold}[1]{\mathbf{#1}}\left(\begin{array}{rrrr} 1 & 3 & -2 & 5 \\ 0 & 4 & -12 & 8 \end{array}\right)$ sage: type(Av) $\newcommand{\Bold}[1]{\mathbf{#1}}\verb|<type|\phantom{\verb!x!}\verb|'sage.matrix.matrix_integer_dense.Matrix_integer_dense'>|$ sage: Av = Av.change_ring(QQ) sage: type(Av); Av.echelon_form() $\newcommand{\Bold}[1]{\mathbf{#1}}\verb|<type|\phantom{\verb!x!}\verb|'sage.matrix.matrix_rational_dense.Matrix_rational_dense'>|$ $\newcommand{\Bold}[1]{\mathbf{#1}}\left(\begin{array}{rrrr} 1 & 0 & 7 & -1 \\ 0 & 1 & -3 & 2 \end{array}\right)$

Playing with finite groups

Unckeck the Typeset box now, please.

sage: S4 = SymmetricGroup(4) sage: S4 Symmetric group of order 4! as a permutation group sage: show(S4) $\newcommand{\Bold}[1]{\mathbf{#1}}\langle (1,2,3,4), (1,2) \rangle$ sage: S4.conjugacy_classes_representatives() [(), (1,2), (1,2)(3,4), (1,2,3), (1,2,3,4)] sage: len(S4.conjugacy_classes_representatives()), len(S4.subgroups()), len(S4.conjugacy_classes_subgroups()), len(S4.normal_subgroups()) (5, 30, 11, 4) sage: u = S4( (1,2,3,4) ) sage: v = S4( (2,4) ) sage: w = S4( ((1,2),(3,4)) ) sage: u.order(), u*v, w==u^-1*v (4, (1,4)(2,3), True) sage: G = S4.subgroup([u, v]) sage: G.order(), G.is_abelian(), G.center().order(), G.is_isomorphic(DihedralGroup(4)) (8, False, 2, True) sage: (1,4) in G, (1,3) in G (False, True) sage: G.cayley_graph(generators=[u, v]).show(color_by_label=True)

sage: G.cayley_graph(generators=[v, w]).show(color_by_label=True)

sage: T = G.cayley_table() sage: T * a b c d e f g h +---------------- a| a b c d e f g h b| b a d c f e h g c| c g a e d h b f d| d h b f c g a e e| e f g h a b c d f| f e h g b a d c g| g c e a h d f b h| h d f b g c e a sage: T.translation() {'a': (), 'c': (1,2)(3,4), 'b': (2,4), 'e': (1,3), 'd': (1,2,3,4), 'g': (1,4,3,2), 'f': (1,3)(2,4), 'h': (1,4)(2,3)} sage: from sage.matrix.operation_table import OperationTable sage: def commutator(h, g): return h*g*h^-1*g^-1 sage: OperationTable(G, commutator) . a b c d e f g h +---------------- a| a a a a a a a a b| a a f f a a f f c| a f a f f a f a d| a f f a f a a f e| a a f f a a f f f| a a a a a a a a g| a f f a f a a f h| a f a f f a f a

Minimal free resolutions of monomial ideals

sage: R. = PolynomialRing(QQ) sage: I = R.ideal([a*c, b*d, a*e, d*e]) sage: R; I Multivariate Polynomial Ring in a, b, c, d, e over Rational Field Ideal (a*c, b*d, a*e, d*e) of Multivariate Polynomial Ring in a, b, c, d, e over Rational Field sage: I.syzygy_module() [ -e 0 c 0] [ 0 -e 0 b] [ 0 0 -d a] [-b*d a*c 0 0] sage: singular.mres(I, 0) [1]: _[1]=d*e _[2]=a*e _[3]=b*d _[4]=a*c [2]: _[1]=c*gen(2)-e*gen(4) _[2]=b*gen(1)-e*gen(3) _[3]=a*gen(1)-d*gen(2) _[4]=a*c*gen(3)-b*d*gen(4) [3]: _[1]=a*c*gen(2)-b*c*gen(3)-b*d*gen(1)+e*gen(4) [4]: _[1]=0 [5]: _[1]=gen(1)