Approximate the solutions as a series

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I would like to solve the following equation $y^2=x^2+ax^2y^2+by^2x^3+cy^3x^2$ where $a,b,c$ are small, so $yapprox x+O(x^3)$. I would like to have a series approximation of the solution rather than an exact one, i.e. like $y=x+c_1x^3+c_2x^4+...$.

Is there a default function in mathematica that does that?

asked 3 hours ago

Shadumu

1085

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up vote
1
down vote

favorite

Is there a default function in mathematica that does that?

asked 3 hours ago

Shadumu

1085

add a commentÂ |Â

up vote
1
down vote

favorite

Is there a default function in mathematica that does that?

asked 3 hours ago

Shadumu

1085

Is there a default function in mathematica that does that?

equation-solving series-expansion approximation

asked 3 hours ago

Shadumu

1085

asked 3 hours ago

Shadumu

1085

asked 3 hours ago

Shadumu

1085

asked 3 hours ago

Shadumu

1085

asked 3 hours ago

Shadumu

1085

add a commentÂ |Â

2 Answers
2

active

oldest

votes

up vote
2
down vote

accepted

Quick-and-dirty solution:

y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) /. 
 y -> x + c1 x^2 + c2 x^3 + c3 x^4 + O[x]^5 // FullSimplify
Solve[% == 0, c1, c2, c3]
y -> x + c1 x^2 + c2 x^3 + c3 x^4 + O[x]^5 /. % // TeXForm

$$yto x+fraca x^32+frac12 (b+c) x^4+Oleft(x^5right)$$

Compare to the exact solution:

FullSimplify[
 Series[
 Solve[y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) == 0, y][[3, 1, 2]]
 , x, 0, 4],
c > 0
]

edited 2 hours ago

answered 2 hours ago

AccidentalFourierTransform

4,8361940

thanks a lot! I'm new to mathematica, would you briefly explain to me the difference between your two methods? and what does the [3,1,2] mean?
â€“Â Shadumu
1 hour ago

add a commentÂ |Â

up vote
2
down vote

Could use implicit differentiation, solve for all derivatives at x==0.

ee = y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) /. y -> y[x];
dpolys = Table[D[ee, x, j], j, 0, 5] /. x -> 0

(* y[0]^2, 2*y[0]*Derivative[1][y][0], -2 - 2*a*y[0]^2 - 2*c*y[0]^3 + 
 2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0], 
 -6*b*y[0]^2 - 12*a*y[0]*Derivative[1][y][0] - 
 18*c*y[0]^2*Derivative[1][y][0] + 
 6*Derivative[1][y][0]*Derivative[2][y][0] + 
 2*y[0]*Derivative[3][y][0], 
 -48*b*y[0]*Derivative[1][y][0] + 6*Derivative[2][y][0]^2 - 
 12*a*(2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0]) - 
 12*c*(6*y[0]*Derivative[1][y][0]^2 + 
 3*y[0]^2*Derivative[2][y][0]) + 
 8*Derivative[1][y][0]*Derivative[3][y][0] + 
 2*y[0]*Derivative[4][y][0], 
 -60*b*(2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0]) + 
 20*Derivative[2][y][0]*Derivative[3][y][0] - 
 20*a*(6*Derivative[1][y][0]*Derivative[2][y][0] + 
 2*y[0]*Derivative[3][y][0]) - 
 20*c*(6*Derivative[1][y][0]^3 + 
 18*y[0]*Derivative[1][y][0]*Derivative[2][y][0] + 
 3*y[0]^2*Derivative[3][y][0]) + 
 10*Derivative[1][y][0]*Derivative[4][y][0] + 
 2*y[0]*Derivative[5][y][0] *)

soln = Solve[dpolys == 0]

(* Out[139]= y[0] -> 0, Derivative[1][y][0] -> -1, 
 Derivative[2][y][0] -> 0, 
 Derivative[3][y][0] -> -3*a, Derivative[4][y][0] -> 12*(-b + c), 
 y[0] -> 0, Derivative[1][y][0] -> 1, Derivative[2][y][0] -> 0, 
 Derivative[3][y][0] -> 3*a, Derivative[4][y][0] -> 12*(b + c) *)

So two solutions, both getting up to the fourth order term. We create a table, then use the solutions to fill in values and recast as a Taylor polynomial for each solution branch.

derivs = Table[D[y[x], x, j], j, 0, 4] /. x -> 0;
derivs.(x^Range[0, 4]/Factorial[Range[0, 4]]) /. soln

(* Out[142]= -x - (a x^3)/2 + 1/2 (-b + c) x^4, 
 x + (a x^3)/2 + 1/2 (b + c) x^4 *)

answered 34 mins ago

Daniel Lichtblau

45.8k275158

add a commentÂ |Â

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

up vote
2
down vote

accepted

Quick-and-dirty solution:

y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) /. 
 y -> x + c1 x^2 + c2 x^3 + c3 x^4 + O[x]^5 // FullSimplify
Solve[% == 0, c1, c2, c3]
y -> x + c1 x^2 + c2 x^3 + c3 x^4 + O[x]^5 /. % // TeXForm

$$yto x+fraca x^32+frac12 (b+c) x^4+Oleft(x^5right)$$

Compare to the exact solution:

FullSimplify[
 Series[
 Solve[y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) == 0, y][[3, 1, 2]]
 , x, 0, 4],
c > 0
]

edited 2 hours ago

answered 2 hours ago

AccidentalFourierTransform

4,8361940

thanks a lot! I'm new to mathematica, would you briefly explain to me the difference between your two methods? and what does the [3,1,2] mean?
â€“Â Shadumu
1 hour ago

add a commentÂ |Â

up vote
2
down vote

accepted

Quick-and-dirty solution:

y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) /. 
 y -> x + c1 x^2 + c2 x^3 + c3 x^4 + O[x]^5 // FullSimplify
Solve[% == 0, c1, c2, c3]
y -> x + c1 x^2 + c2 x^3 + c3 x^4 + O[x]^5 /. % // TeXForm

$$yto x+fraca x^32+frac12 (b+c) x^4+Oleft(x^5right)$$

Compare to the exact solution:

FullSimplify[
 Series[
 Solve[y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) == 0, y][[3, 1, 2]]
 , x, 0, 4],
c > 0
]

edited 2 hours ago

answered 2 hours ago

AccidentalFourierTransform

4,8361940

thanks a lot! I'm new to mathematica, would you briefly explain to me the difference between your two methods? and what does the [3,1,2] mean?
â€“Â Shadumu
1 hour ago

add a commentÂ |Â

up vote
2
down vote

accepted

Quick-and-dirty solution:

y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) /. 
 y -> x + c1 x^2 + c2 x^3 + c3 x^4 + O[x]^5 // FullSimplify
Solve[% == 0, c1, c2, c3]
y -> x + c1 x^2 + c2 x^3 + c3 x^4 + O[x]^5 /. % // TeXForm

$$yto x+fraca x^32+frac12 (b+c) x^4+Oleft(x^5right)$$

Compare to the exact solution:

FullSimplify[
 Series[
 Solve[y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) == 0, y][[3, 1, 2]]
 , x, 0, 4],
c > 0
]

edited 2 hours ago

answered 2 hours ago

AccidentalFourierTransform

4,8361940

Quick-and-dirty solution:

y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) /. 
 y -> x + c1 x^2 + c2 x^3 + c3 x^4 + O[x]^5 // FullSimplify
Solve[% == 0, c1, c2, c3]
y -> x + c1 x^2 + c2 x^3 + c3 x^4 + O[x]^5 /. % // TeXForm

$$yto x+fraca x^32+frac12 (b+c) x^4+Oleft(x^5right)$$

Compare to the exact solution:

FullSimplify[
 Series[
 Solve[y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) == 0, y][[3, 1, 2]]
 , x, 0, 4],
c > 0
]

edited 2 hours ago

answered 2 hours ago

AccidentalFourierTransform

4,8361940

edited 2 hours ago

answered 2 hours ago

AccidentalFourierTransform

4,8361940

answered 2 hours ago

AccidentalFourierTransform

4,8361940

answered 2 hours ago

AccidentalFourierTransform

4,8361940

thanks a lot! I'm new to mathematica, would you briefly explain to me the difference between your two methods? and what does the [3,1,2] mean?
â€“Â Shadumu
1 hour ago

add a commentÂ |Â

thanks a lot! I'm new to mathematica, would you briefly explain to me the difference between your two methods? and what does the [3,1,2] mean?
â€“Â Shadumu
1 hour ago

thanks a lot! I'm new to mathematica, would you briefly explain to me the difference between your two methods? and what does the [3,1,2] mean?
â€“Â Shadumu
1 hour ago

add a commentÂ |Â

up vote
2
down vote

Could use implicit differentiation, solve for all derivatives at x==0.

ee = y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) /. y -> y[x];
dpolys = Table[D[ee, x, j], j, 0, 5] /. x -> 0

(* y[0]^2, 2*y[0]*Derivative[1][y][0], -2 - 2*a*y[0]^2 - 2*c*y[0]^3 + 
 2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0], 
 -6*b*y[0]^2 - 12*a*y[0]*Derivative[1][y][0] - 
 18*c*y[0]^2*Derivative[1][y][0] + 
 6*Derivative[1][y][0]*Derivative[2][y][0] + 
 2*y[0]*Derivative[3][y][0], 
 -48*b*y[0]*Derivative[1][y][0] + 6*Derivative[2][y][0]^2 - 
 12*a*(2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0]) - 
 12*c*(6*y[0]*Derivative[1][y][0]^2 + 
 3*y[0]^2*Derivative[2][y][0]) + 
 8*Derivative[1][y][0]*Derivative[3][y][0] + 
 2*y[0]*Derivative[4][y][0], 
 -60*b*(2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0]) + 
 20*Derivative[2][y][0]*Derivative[3][y][0] - 
 20*a*(6*Derivative[1][y][0]*Derivative[2][y][0] + 
 2*y[0]*Derivative[3][y][0]) - 
 20*c*(6*Derivative[1][y][0]^3 + 
 18*y[0]*Derivative[1][y][0]*Derivative[2][y][0] + 
 3*y[0]^2*Derivative[3][y][0]) + 
 10*Derivative[1][y][0]*Derivative[4][y][0] + 
 2*y[0]*Derivative[5][y][0] *)

soln = Solve[dpolys == 0]

(* Out[139]= y[0] -> 0, Derivative[1][y][0] -> -1, 
 Derivative[2][y][0] -> 0, 
 Derivative[3][y][0] -> -3*a, Derivative[4][y][0] -> 12*(-b + c), 
 y[0] -> 0, Derivative[1][y][0] -> 1, Derivative[2][y][0] -> 0, 
 Derivative[3][y][0] -> 3*a, Derivative[4][y][0] -> 12*(b + c) *)

So two solutions, both getting up to the fourth order term. We create a table, then use the solutions to fill in values and recast as a Taylor polynomial for each solution branch.

derivs = Table[D[y[x], x, j], j, 0, 4] /. x -> 0;
derivs.(x^Range[0, 4]/Factorial[Range[0, 4]]) /. soln

(* Out[142]= -x - (a x^3)/2 + 1/2 (-b + c) x^4, 
 x + (a x^3)/2 + 1/2 (b + c) x^4 *)

answered 34 mins ago

Daniel Lichtblau

45.8k275158

add a commentÂ |Â

up vote
2
down vote

Could use implicit differentiation, solve for all derivatives at x==0.

ee = y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) /. y -> y[x];
dpolys = Table[D[ee, x, j], j, 0, 5] /. x -> 0

(* y[0]^2, 2*y[0]*Derivative[1][y][0], -2 - 2*a*y[0]^2 - 2*c*y[0]^3 + 
 2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0], 
 -6*b*y[0]^2 - 12*a*y[0]*Derivative[1][y][0] - 
 18*c*y[0]^2*Derivative[1][y][0] + 
 6*Derivative[1][y][0]*Derivative[2][y][0] + 
 2*y[0]*Derivative[3][y][0], 
 -48*b*y[0]*Derivative[1][y][0] + 6*Derivative[2][y][0]^2 - 
 12*a*(2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0]) - 
 12*c*(6*y[0]*Derivative[1][y][0]^2 + 
 3*y[0]^2*Derivative[2][y][0]) + 
 8*Derivative[1][y][0]*Derivative[3][y][0] + 
 2*y[0]*Derivative[4][y][0], 
 -60*b*(2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0]) + 
 20*Derivative[2][y][0]*Derivative[3][y][0] - 
 20*a*(6*Derivative[1][y][0]*Derivative[2][y][0] + 
 2*y[0]*Derivative[3][y][0]) - 
 20*c*(6*Derivative[1][y][0]^3 + 
 18*y[0]*Derivative[1][y][0]*Derivative[2][y][0] + 
 3*y[0]^2*Derivative[3][y][0]) + 
 10*Derivative[1][y][0]*Derivative[4][y][0] + 
 2*y[0]*Derivative[5][y][0] *)

soln = Solve[dpolys == 0]

(* Out[139]= y[0] -> 0, Derivative[1][y][0] -> -1, 
 Derivative[2][y][0] -> 0, 
 Derivative[3][y][0] -> -3*a, Derivative[4][y][0] -> 12*(-b + c), 
 y[0] -> 0, Derivative[1][y][0] -> 1, Derivative[2][y][0] -> 0, 
 Derivative[3][y][0] -> 3*a, Derivative[4][y][0] -> 12*(b + c) *)

So two solutions, both getting up to the fourth order term. We create a table, then use the solutions to fill in values and recast as a Taylor polynomial for each solution branch.

derivs = Table[D[y[x], x, j], j, 0, 4] /. x -> 0;
derivs.(x^Range[0, 4]/Factorial[Range[0, 4]]) /. soln

(* Out[142]= -x - (a x^3)/2 + 1/2 (-b + c) x^4, 
 x + (a x^3)/2 + 1/2 (b + c) x^4 *)

answered 34 mins ago

Daniel Lichtblau

45.8k275158

add a commentÂ |Â

up vote
2
down vote

Could use implicit differentiation, solve for all derivatives at x==0.

ee = y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) /. y -> y[x];
dpolys = Table[D[ee, x, j], j, 0, 5] /. x -> 0

(* y[0]^2, 2*y[0]*Derivative[1][y][0], -2 - 2*a*y[0]^2 - 2*c*y[0]^3 + 
 2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0], 
 -6*b*y[0]^2 - 12*a*y[0]*Derivative[1][y][0] - 
 18*c*y[0]^2*Derivative[1][y][0] + 
 6*Derivative[1][y][0]*Derivative[2][y][0] + 
 2*y[0]*Derivative[3][y][0], 
 -48*b*y[0]*Derivative[1][y][0] + 6*Derivative[2][y][0]^2 - 
 12*a*(2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0]) - 
 12*c*(6*y[0]*Derivative[1][y][0]^2 + 
 3*y[0]^2*Derivative[2][y][0]) + 
 8*Derivative[1][y][0]*Derivative[3][y][0] + 
 2*y[0]*Derivative[4][y][0], 
 -60*b*(2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0]) + 
 20*Derivative[2][y][0]*Derivative[3][y][0] - 
 20*a*(6*Derivative[1][y][0]*Derivative[2][y][0] + 
 2*y[0]*Derivative[3][y][0]) - 
 20*c*(6*Derivative[1][y][0]^3 + 
 18*y[0]*Derivative[1][y][0]*Derivative[2][y][0] + 
 3*y[0]^2*Derivative[3][y][0]) + 
 10*Derivative[1][y][0]*Derivative[4][y][0] + 
 2*y[0]*Derivative[5][y][0] *)

soln = Solve[dpolys == 0]

(* Out[139]= y[0] -> 0, Derivative[1][y][0] -> -1, 
 Derivative[2][y][0] -> 0, 
 Derivative[3][y][0] -> -3*a, Derivative[4][y][0] -> 12*(-b + c), 
 y[0] -> 0, Derivative[1][y][0] -> 1, Derivative[2][y][0] -> 0, 
 Derivative[3][y][0] -> 3*a, Derivative[4][y][0] -> 12*(b + c) *)

So two solutions, both getting up to the fourth order term. We create a table, then use the solutions to fill in values and recast as a Taylor polynomial for each solution branch.

derivs = Table[D[y[x], x, j], j, 0, 4] /. x -> 0;
derivs.(x^Range[0, 4]/Factorial[Range[0, 4]]) /. soln

(* Out[142]= -x - (a x^3)/2 + 1/2 (-b + c) x^4, 
 x + (a x^3)/2 + 1/2 (b + c) x^4 *)

answered 34 mins ago

Daniel Lichtblau

45.8k275158

Could use implicit differentiation, solve for all derivatives at x==0.

ee = y^2 - (x^2 + a x^2 y^2 + b y^2 x^3 + c y^3 x^2) /. y -> y[x];
dpolys = Table[D[ee, x, j], j, 0, 5] /. x -> 0

(* y[0]^2, 2*y[0]*Derivative[1][y][0], -2 - 2*a*y[0]^2 - 2*c*y[0]^3 + 
 2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0], 
 -6*b*y[0]^2 - 12*a*y[0]*Derivative[1][y][0] - 
 18*c*y[0]^2*Derivative[1][y][0] + 
 6*Derivative[1][y][0]*Derivative[2][y][0] + 
 2*y[0]*Derivative[3][y][0], 
 -48*b*y[0]*Derivative[1][y][0] + 6*Derivative[2][y][0]^2 - 
 12*a*(2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0]) - 
 12*c*(6*y[0]*Derivative[1][y][0]^2 + 
 3*y[0]^2*Derivative[2][y][0]) + 
 8*Derivative[1][y][0]*Derivative[3][y][0] + 
 2*y[0]*Derivative[4][y][0], 
 -60*b*(2*Derivative[1][y][0]^2 + 2*y[0]*Derivative[2][y][0]) + 
 20*Derivative[2][y][0]*Derivative[3][y][0] - 
 20*a*(6*Derivative[1][y][0]*Derivative[2][y][0] + 
 2*y[0]*Derivative[3][y][0]) - 
 20*c*(6*Derivative[1][y][0]^3 + 
 18*y[0]*Derivative[1][y][0]*Derivative[2][y][0] + 
 3*y[0]^2*Derivative[3][y][0]) + 
 10*Derivative[1][y][0]*Derivative[4][y][0] + 
 2*y[0]*Derivative[5][y][0] *)

soln = Solve[dpolys == 0]

(* Out[139]= y[0] -> 0, Derivative[1][y][0] -> -1, 
 Derivative[2][y][0] -> 0, 
 Derivative[3][y][0] -> -3*a, Derivative[4][y][0] -> 12*(-b + c), 
 y[0] -> 0, Derivative[1][y][0] -> 1, Derivative[2][y][0] -> 0, 
 Derivative[3][y][0] -> 3*a, Derivative[4][y][0] -> 12*(b + c) *)

So two solutions, both getting up to the fourth order term. We create a table, then use the solutions to fill in values and recast as a Taylor polynomial for each solution branch.

derivs = Table[D[y[x], x, j], j, 0, 4] /. x -> 0;
derivs.(x^Range[0, 4]/Factorial[Range[0, 4]]) /. soln

(* Out[142]= -x - (a x^3)/2 + 1/2 (-b + c) x^4, 
 x + (a x^3)/2 + 1/2 (b + c) x^4 *)

answered 34 mins ago

Daniel Lichtblau

45.8k275158

answered 34 mins ago

Daniel Lichtblau

45.8k275158

answered 34 mins ago

Daniel Lichtblau

45.8k275158

answered 34 mins ago

Daniel Lichtblau

45.8k275158

add a commentÂ |Â

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