Chapter 8: Infinite Sequences and Series
Section 8.5: Taylor Series
Example 8.5.17
Obtain p1x and p2x, degree 8 Maclaurin polynomials for fx=ex and gx=ln1+x, respectively.
Form the product p1⋅p2.
Obtain the degree 11 Maclaurin polynomial for the product fx⋅gx.
Obtain the first 10 coefficients cn formed from the Cauchy product of the coefficients of p1 and p2.
Observe that the coefficients formed from the Cauchy product (Theorem 8.2.4) are always correct, but those formed from the product of the polynomials are correct only to a limited degree in x.
Solution
Part (a)
Obtain the Maclaurin polynomials p1x and p2x
Write each function fx and gx.
Context Panel: Series≻Series≻x Complete the Series dialog as per Figure 8.5.17(a).
Context Panel: Assign to a Name≻p[1] or p[2]
Figure 8.5.17(a) Series dialogc
ⅇx→series in x1+x+12⁢x2+16⁢x3+124⁢x4+1120⁢x5+1720⁢x6+15040⁢x7+140320⁢x8→assign to a namep1
ln1+x→series in xx−12⁢x2+13⁢x3−14⁢x4+15⁢x5−16⁢x6+17⁢x7−18⁢x8→assign to a namep2
Part (b)
Form the (sorted) product p1x⋅p2x
Although the expanded product can be obtained via the Context Panel, the sort option in the Context Panel does not provide for sorting with the powers ascending. Hence, the sort and expand commands have been applied directly.
sortexpandp1⋅p2,x,ascending
x+12⁢x2+13⁢x3+340⁢x5−7144⁢x6+23504⁢x7−29720⁢x8−129117280⁢x9−535391209600⁢x10−9697604800⁢x11−1457345600⁢x12−527604800⁢x13−2531693440⁢x14−147040⁢x15−1322560⁢x16
Part (c)
Obtain the degree 11 Maclaurin polynomial for the product fx⋅gx
Write the product ex ln1+x, being sure to use the exponential "e" taken from one of the palettes, or from Command Completion.
Context Panel: Series≻Series≻x (See Figure 8.5.17(a) for guidance completing the Series dialog.)
ⅇx⋅ln1+x→series in xx+12⁢x2+13⁢x3+340⁢x5−7144⁢x6+23504⁢x7−29720⁢x8+62917280⁢x9−1202873628800⁢x10+60733719958400⁢x11
Part (d)
Form the Cauchy product of the coefficients of the series for fx and gx
To obtain expressions for the coefficients of the series for f and g, obtain their formal power series.
Write ex, being sure to use Maple's exponential "e".
Context Panel: Series≻Formal Power Series
ⅇx→formal series∑n=0∞⁡xnn!
Write ln1+x.
ln1+x→formal series∑n=0∞⁡−1n⁢xn+1n+1
Write the series for g as x∑n=0∞−1nn+1xn so that the powers in the series for both f and g agree.
Write an and bn as Maple functions
Write an=… Context Panel: Assign Function
an=1/n!→assign as functiona
Write bn=… Context Panel: Assign Function
bn=−1n/n+1→assign as functionb
Form cn, the coefficient of the Cauchy product, as a Maple function
Write cn=… Context Panel: Assign Function
cn=∑k=0nak⋅bn−k→assign as functionc
Display a sequence of coefficients cn
Write cn
Context Panel: Sequence≻n Complete the Sequence dialog as per Figure 8.5.17(b)ib
Context Panel: Sequence≻n Complete the Sequence dialog as per Figure 8.5.17(b)
Context Panel: Simplify≻Simplify
Figure 8.5.17(b) Sequence dialog
cn→sequence w.r.t. n1,hypergeom⁡−1,1,1,2,1,hypergeom⁡−2,1,1,2,12,hypergeom⁡−3,1,1,2,16,hypergeom⁡−4,1,1,2,124,hypergeom⁡−5,1,1,2,1120,hypergeom⁡−6,1,1,2,1720,hypergeom⁡−7,1,1,2,15040,hypergeom⁡−8,1,1,2,140320,hypergeom⁡−9,1,1,2,1362880,hypergeom⁡−10,1,1,2,13628800= simplify 1,12,13,0,340,−7144,23504,−29720,62917280,−1202873628800,60733719958400
Part (e)
The coefficients formed via the Cauchy product are all correct, as a comparison with the Maclaurin series for the product fx⋅gx shows. However, the coefficients in the product of p1x⋅p2x are correct only up to the term of degree 8. When multiplying Taylor or Maclaurin polynomials, care should be taken with respect to the degree of the polynomials. Not all the terms in the finite product will be correct.
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