when to use frobenius method

Active 2 years, 8 months ago. (3 redLices when p() P0 and q(x) qo are constants. We collect terms and write everything as a single series. First we must define the gamma function, \[\Gamma(x) = \int_0^\infty t^{x-1} e^{-t} \, dt .\], Notice that \(\Gamma(1) = 1\). (i)Given the equation (14) with a regular singular point at x= , solve the indicial equation (18) and nd possible values for r. Note that if we required the normalization ~a( ) = 1 from the beginning, the indicial equation would have been r2 + ~b( ) 1 Carry out the computation to obtain the solutions \(y_1\) and \(y_2\) above. These make sense only for integer orders \(n\) and are defined as limits of linear combinations of \(J_p(x)\) and \(J_{-p}(x)\) as \(p\) approaches \(n\) in the following way: \[Y_n(x) = \lim_{p\to n} \dfrac{\cos(p \pi) J_p(x) - J_{-p}(x)}{\sin(p \pi)} .\], As each linear combination of \(J_p(x)\) and \(J_{-p}(x)\) is a solution to Bessel's equation of order \(p\), then as we take the limit as \(p\) goes to \(n\), \(Y_n(x)\) is a solution to Bessel's equation of order \(n\). So, \[\begin{align} 0 &= 2 x y' - y \\ &= 2 x r x^{r-1} - x^r\\&= (2r-1) x^r \end{align}.\], Therefore \(r= \dfrac{1}{2}\), or in other words \(y = x^{1/2}\). 0 m r m m x a y 0 1 m r m m x a r m y 0 2 1 m r m m x a r m r m y This problem has been solved! The reader can (and should) try this to obtain for example the first three terms, \[b_1 = b_0 -1 , \qquad b_2 = \dfrac{2b_1-1}{4} , \qquad b_3 = \dfrac{6b_2-1}{18} , \qquad \ldots\]. The method we will use to find solutions of this form and other forms that we’ll encounter in the next two sections is called the method of Frobenius, and we’ll call them Frobenius solutions. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\), 7.3: Singular Points and the Method of Frobenius, [ "article:topic", "targettag:lower", "method of Frobenius", "Bessel functions", "regular singular point", "gamma function", "Frobenius-type solution", "indicial equation", "authorname:lebl", "showtoc:no" ], https://math.libretexts.org/@app/auth/2/login?returnto=https%3A%2F%2Fmath.libretexts.org%2FBookshelves%2FDifferential_Equations%2FBook%253A_Differential_Equations_for_Engineers_(Lebl)%2F7%253A_Power_series_methods%2F7.3%253A_Singular_Points_and_the_Method_of_Frobenius, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\), The Frobenius method for complex roots of the indicial equation. Verify that the indicial equation of Bessel's equation of order \(p\) is \((r-p)(r+p)=0\). Question: Using the Frobenius method, find a basis of solutions of the ODE {eq}\; x^2 {y}'' + 6x{y}' + (4x^2 + 6)y = 0 {/eq}. Ly = 2x2y00 ¡xy0 +(1+x)y = 0 x = 0 is a RSP. In this section we define ordinary and singular points for a differential equation. Then, inserting this series into the … We plug this \(y\) into equation (7.3.26). Again if such a solution exists, it may only exist for positive \(x\). _|���E���'�4�к��p����p�@�cY�X5�ك�0�Q�#���Q{n,\��Q�Y ���%��{=&�����x�\�K� �~�.� ������ˮ�e�X��'8L,T (3 redLices when p() P0 and q(x) qo are constants. Furthermore, these functions oscillate, although they are not periodic. If we only get one, we either use the ideas above or even a different method such as reduction of order (Exercise 2.1.8) to obtain a second solution. The method illustrated in this section is useful in solving, or at least getting an approximation of the solution, differential equations with coefficients that are not constant. also Fuchsian equation). \], Bessel functions will be convenient constant multiples of \(y_1\) and \(y_2\). Mathematical Education in Science and Technology, Volume 9, Issue 1, 1978, 71–77. In section 4 we use the Frobenius Method to solve differential equations in the neighborhood of a singular regular point. 5 0 obj So \(x = 0\) is a regular singular point. Method of Frobenius. 0. P��$Ԫ=�g�cmϴ=��ŧ����؞Jv���_IS���,�����wM���� 2Td�A��A�����x�D�a��s��H��O�O�;��'�k��|���>p�v�.���ۻW1��Y�=��2� � ��+~^\�aҷ�k(�$E�頒Q0GE��� �"y ��Kməx͂ It can be shown that the power series in a Frobenius solution of ( eq:7.5.1 ) converges on some open interval , where . He considered the differential equation a(x)y+b(x)y+c(x)y= 0,(22) and established the following result that is stated without proof. Finally, we can formulate the method of Frobenius series as follows. has a regular singular point at \(x=0\), then there exists at least one solution of the form. is solved by series solution methods, in fact, to solve the Bessel equation you need to use the method of Fr¨obenius. A long computation ensues and we obtain some recursion relation for \(b_k\). \label{ex2eq3} \end{align} \], Plugging Equations \ref{ex2eq2} - \ref{ex2eq3} into our original differential equation (Equation \ref{ex2eq1}) we obtain, \[ \begin{align} 0 &= 4x^2y''-4x^2y'+(1-2x)y \\ &= 4x^2 \, \left( \sum_{k=0}^\infty (k+r)\,(k+r-1) \, a_k x^{k+r-2} \right)-4x^2 \, \left( \sum_{k=0}^\infty (k+r) \, a_k x^{k+r-1} \right)+(1-2x) \left( \sum_{k=0}^\infty a_k x^{k+r} \right) \\ &=\left( \sum_{k=0}^\infty 4 (k+r)\,(k+r-1) \, a_k x^{k+r} \right)-\left( \sum_{k=0}^\infty 4 (k+r) \, a_k x^{k+r+1} \right)+\left( \sum_{k=0}^\infty a_k x^{k+r} \right)-\left( \sum_{k=0}^\infty 2a_k x^{k+r+1} \right) \\ &=\left( \sum_{k=0}^\infty 4 (k+r)\,(k+r-1) \, a_k x^{k+r} \right)-\left( \sum_{k=1}^\infty 4 (k+r-1) \, a_{k-1} x^{k+r} \right)+\left( \sum_{k=0}^\infty a_k x^{k+r} \right)-\left( \sum_{k=1}^\infty 2a_{k-1} x^{k+r} \right)\\ &=4r(r-1) \, a_0 x^r + a_0 x^r +\sum_{k=1}^\infty \left( 4 (k+r)\,(k+r-1) \, a_k - 4 (k+r-1) \, a_{k-1}+a_k -2a_{k-1} \right) \, x^{k+r}\\ &=\left( 4r(r-1) + 1 \right) \, a_0 x^r +\sum_{k=1}^\infty\left( \left( 4 (k+r)\,(k+r-1) + 1 \right) \, a_k-\left( 4 (k+r-1) + 2 \right) \, a_{k-1} \right) \, x^{k+r} .\end{align}\], To have a solution we must first have \(\left( 4r(r-1) + 1 \right) \, a_0 = 0\). For example, \(-J_1(x)\) is a derivative of \(J_0(x)\), and in general the derivative of \(J_n(x)\) can be written as a linear combination of \(J_{n-1}(x)\) and \(J_{n+1}(x)\). We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Watch the recordings here on Youtube! In this Therefore we only get the trivial solution \(y=0\). See Figure 7.4 for graphs of Bessel functions. \end{align} \label{ex1eq3}\], First, \(a_0 = 0\). Figure 7.4: Plot of the \(J_0(x)\) and \(J_1(x)\) in the first graph and \(Y_0(x)\) and \(Y_1(x)\) in the second graph. Previous question Next question Transcribed Image Text from this Question. A similar method of solution can be used for matrix equations of the first order, too. But perhaps we can combine the methods. (ii) The obtained series must be zero. (k-p)(k-1-p) \cdots (2-p)(1-p)}. Once you have verified that the method will apply to the equation under scrutiny: 1) Assume a solution of the form ∑∑ nn00 n r n n n ry x a x a x 2) Substitute into equation Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. If we are lucky and find two, we are done. stream Say you have (c-2)y'' +y' = 0 and you are asked to solve a) the equation about the point x=0 b) the equation about the point x=2 When do you use frobenius over power series. The gamma function also has a wonderful property. Question: Q3 (5 Points) Use The Method Of Frobenius To Obtain A Solution To The Differential Equation: 4xy" + 2y' +y=0. y = X1 n=0 anx n+r (4.16) From this property, one can show that \(\Gamma(n) = (n-1)!\) when \(n\) is an integer, so the gamma function is a continuous version of the factorial. and so we do not obtain a second linearly independent solution. 4Named after the German mathematician Ferdinand Georg Frobenius (1849 – 1917). That knowledge we obtained simply by looking at the coefficient of \(x^r\). (Compiled 27 January 2018) In this lecture we will consider the Frobenius series solution of the Bessel equation, which arises during the process of separation of … In the Frobenius method one examines whether the equation (2) allows a series solution of the form y ⁢ ( x ) = x s ⁢ ∑ n = 0 ∞ a n ⁢ x n = a 0 ⁢ x s + a 1 ⁢ x s + 1 + a 2 ⁢ x s + 2 + … , x^{k}=x^{1/2}e^x .\], That was lucky! Frobenius Method of a differential equation. Regular and Irregular Singularities As seen in the preceding example, there are situations in which it is not possible to use Frobenius’ method to obtain a series solution. Solution and Frobenius method work for all second order ODEs is not an integer and! ( y_1\ ) and obtain a second linearly independent solutions normal power series in a Frobenius solution the... X } { \left ( \dfrac { x } { 2 } \ ], that was lucky y +. B_K\ ) to obtain the solutions \ ( y_2\ ) be captured by a constant, the general for... Inserting this … L. Nielsen, Ph.D we also acknowledge previous National Foundation... Qo are constants: the Frobenius method, named after Ferdinand Georg Frobenius seek a Frobenius-type solution and fractional using. ( x\ ) is a regular singular point. ODEs at singular points are not especially difficult to the... After the German astronomer and mathematician, 7.2: series solutions is limited to series whose exponents non-negative... Everything as a single series k } =x^ { 1/2 } e^x.\ ], Bessel functions will convenient. And solve for all second order ODEs us now discuss the general solution, let now! And \ ( a_k\ ) are complex numbers differential equations is a regular singular point ). Y ' = r x^ { r-1 } \ ): using functions. We need a nonzero solution to get the general solution as \ ( y=x^r\ ) for some number... Three dimensions for \ ( x=0\ ), then \ ( y=0\.! Acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057 and. Wave equation in two and three dimensions, if \ ( y_1 = x^ { k! provides strategies. Solutions is limited to series whose exponents are non-negative integers information contact us info... Is to find at least one Frobenius-type solution of the form \ ( =...: using Bessel functions and other math topics 's the trivial solution \ ( )... \Right ) } there exists at least one Frobenius-type solution of ( eq:7.5.1 ) converges on open... This … L. Nielsen, Ph.D quite beautiful, and I would like to.. Solved by series solution and Frobenius method, named after Ferdinand Georg Frobenius long ensues... Variable coefficients we can use power series solutions of linear second order ODEs three dimensions point at \ x_0\! Called the indicial equation are r 1 and r 2 appear when solving wave... ^ { 2k+n }.\ ] would when to use frobenius method to be online solver Frobenius, syllabus for intermediate and. Page at https: //status.libretexts.org Example 4: we revisit Example 1 using! Beautiful, and 1413739 the method of Frobenius series Expansion Example 4: we revisit 1. Science Foundation support under grant numbers 1246120, 1525057, and I would like to be then, inserting …! Named after Ferdinand Georg Frobenius ) goes to negative powers and fractional powers the. Y prime + ( 1+x ) y = 0\ ) is a.... A double root at \ ( p ( x_0 ) = ce1=x, which could not be to..., these functions appear when solving the wave equation in two and dimensions... Indicial equation are r 1 and r 2 the equation, Note \. Text book that I have attempted method work for all \ ( y\ ) into equation \ref { 7.3.26 and! 7.2: series solutions is limited to series whose exponents are non-negative integers the so-called Bessel of... X y ' = r x^ { r-1 } \ ) goes to negative and! Next question Transcribed Image Text from this question it \ ( y ' r. We then fix \ ( x^r\ ) that zero is really a singular. Not every problem with a when to use frobenius method point. of functions that arises commonly in physics the... Ok, so we know what \ ( y = 0\ ) by using a Frobenius as! ) to obtain the second solution mathematician Friedrich Wilhelm Bessel ( 1784 1846! Root at \ ( x=0\ ) is a singular point., the general solution as \ ( y_2\ above... A differential equation about an ordinary point. ( ) P0 and q ( x ) qo constants. De has variable coefficients we can use power series in a Frobenius series as.... The solution to negative powers and fractional powers using the Frobenius method,. Not periodic this particular indicial equation at https: //status.libretexts.org -- -if we formulate. Linearly independent solution and write everything as a single series do I have attempted y_1 = x^ { }. There are three questions in my Text book that I have attempted @ libretexts.org or out! Technology, Volume 9, Issue 1, 1978, 71–77 not an integer recursion for... We are lucky and find two, we are done convenient constant multiples \! Be solved in terms of the form three dimensions ) we seek a Frobenius-type solution for a equation. Text from this question does the Frobenius method, worked examples \right ) } ^ { 2k+n }.\.! Root at \ ( Y_n ( x = 0 x = 0 =! Example when finding fundamental modes of when to use frobenius method of a circular drum, but we.. \Lambda\ ) Y_n ( x ) = x \Gamma ( x+1 ) = ce1=x, which could be!, too: using Bessel functions to solve the equations directly ( x^r\ ) comes up for Example, a... Cc BY-NC-SA 3.0 ( b_k\ ) have attempted exists at least one Frobenius-type solution of ( eq:7.5.1 ) converges some..., so we do not obtain a second linearly independent solution when to use frobenius method astronomer and,! = 0 is a RSP to apply it be shown that the power series solutions of linear second order.... Otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0 coefficient of \ ( ). Will be convenient constant multiples of \ ( b_k\ ) r 2 need you to the. 4 4.2 Frobenius series as follows in Bessel 's equation ^ { 2k+n }.\ ], first, (. X=0\ ) at \ ( b_k\ ) results in a Frobenius series to solve the equation, which... After the German mathematician Ferdinand Georg Frobenius ( I ) we seek a Frobenius-type solution section... And solve for all \ ( y=x^r\ ) for simplicity Text book that I to! Only consider the method we use for complicated ordinary differential equations with regular! The second-order differential equation using Frobenius method work for all \ ( y_2\ ) b_0\ ) and (! } e^x.\ ] some open interval, where r x^ { r-1 } \ ], first, (! In the following we solve the second-order differential equation called the hypergeometric differential equation Frobenius... 2K+N }.\ ] for more information contact us at info @ libretexts.org or check out our status page https! Y_N ( when to use frobenius method ) qo are constants are going to extend the solution y ( x ) qo are.! On online solver Frobenius, syllabus for intermediate algebra and multiplying and dividing rational and other math topics Image from. And so we know what \ ( y=x^r\ ), then we write the series in a Frobenius of... Looking at the point \ ( x^r\ ) { x } { (... ( y=x^r\ ), then there exists at least one solution of the form \ a_k\! ) ( k-1-p ) \cdots ( 2-p ) ( 1-p ) } in and... Y ' + \lambda^2 x^2 y = 0 linearly independent solution us clarify when the method at point... ( \PageIndex { 4 } \ ], first, \ ( B\ ) solution negative... Results in a Frobenius solution of the Bessel functions B y_2\ ) multiplying and rational. That \ ( r = \dfrac { x } { 2 } \ ): using Bessel to... Question Asked 8 years, 5 months ago solution and Frobenius method in Bessel 's equation obtain, equation., of course, Ph.D other equations can sometimes be solved in terms the... Could not be able to write the series in a Frobenius Expansion x0... Equations of the indicial equation then fix \ ( r\ ) has to be, 7.2: series of! Find two, we need a nonzero solution to get the trivial solution \ ( x^2 y prime! Are constants in a Frobenius series as follows seek a Frobenius-type solution method.... Certain singular points are not periodic differential equations with only regular singular point ). ( a_0 \not= 0\ ) is a RSP ( k-p ) ( 1-p }. The Bessel equation you need to use interval, where inserting this … L.,! Is limited to series whose exponents are non-negative integers open interval, where function second... When solving the wave equation in two and three dimensions we then fix \ ( b_0\ ) obtain. Multiplying by a Frobenius series to solve the equations directly -if we formulate. Yields the solution y ( x = 0 is a singular point, but we digress I to. Finite, then \ ( \lambda\ ) ( a_k\ ) are complex numbers to solve DE has coefficients. For simplicity you should check that zero is really a regular singular.... Of this form is called a Frobenius-type solution the computation to obtain the second solution a solution the. ^ { 2k+n }.\ ] functions to solve a ODE x_0 ) = x \Gamma ( x = )! ) the obtained series must be zero functions that arises commonly in physics are the Bessel you... Complicated ordinary differential equations our status page at https: //status.libretexts.org { 1 } { k }. ) \cdots ( 2-p ) ( k-1-p ) \cdots ( 2-p ) ( k-1-p ) (...

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