about the minimax and orthogonal polynomial Exponential function












0














using legendre Polynomial between 0,1 for the exponential function you could get te following series
$$e^{-frac{p}{2}}=sum _{n=0}^{infty } sqrt{frac{pi }{2}} (-1)^{ n} (2 n+1) I_{n+frac{1}{2}}(1) P_nleft(frac{p}{2}right)$$ it is suppose that t is a good aproximation using only m=2 you get the error



$$e^{-frac{p}{2}}simeq -frac{5}{8} sqrt{frac{pi }{2}} left(3 p^2-4right) I_{frac{5}{2}}(1)+frac{3}{2} sqrt{frac{pi }{2}} p I_{frac{3}{2}}(1)+left(-sqrt{frac{pi }{2}}right) I_{frac{1}{2}}(1)$$ for p=1 the error it is about
0.027875
it is suppose that this series could be improve ( maybe get it better improve using chevyshef series) but de following series
$$e^{-frac{p}{2}}simeq frac{3 pi (4 p+7) I_{frac{3}{4}}left(frac{p}{2}right)}{8 p^{3/4} Gamma left(frac{1}{4}right)}-frac{15 pi sqrt[4]{p} I_{-frac{1}{4}}left(frac{p}{2}right)}{8 Gamma left(frac{1}{4}right)}$$
for m=2 (usin two terms) gives te error -0.00286246 10 times better
how it is possible? it is suppose that Legendre it is orthogonal polinomial










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    using legendre Polynomial between 0,1 for the exponential function you could get te following series
    $$e^{-frac{p}{2}}=sum _{n=0}^{infty } sqrt{frac{pi }{2}} (-1)^{ n} (2 n+1) I_{n+frac{1}{2}}(1) P_nleft(frac{p}{2}right)$$ it is suppose that t is a good aproximation using only m=2 you get the error



    $$e^{-frac{p}{2}}simeq -frac{5}{8} sqrt{frac{pi }{2}} left(3 p^2-4right) I_{frac{5}{2}}(1)+frac{3}{2} sqrt{frac{pi }{2}} p I_{frac{3}{2}}(1)+left(-sqrt{frac{pi }{2}}right) I_{frac{1}{2}}(1)$$ for p=1 the error it is about
    0.027875
    it is suppose that this series could be improve ( maybe get it better improve using chevyshef series) but de following series
    $$e^{-frac{p}{2}}simeq frac{3 pi (4 p+7) I_{frac{3}{4}}left(frac{p}{2}right)}{8 p^{3/4} Gamma left(frac{1}{4}right)}-frac{15 pi sqrt[4]{p} I_{-frac{1}{4}}left(frac{p}{2}right)}{8 Gamma left(frac{1}{4}right)}$$
    for m=2 (usin two terms) gives te error -0.00286246 10 times better
    how it is possible? it is suppose that Legendre it is orthogonal polinomial










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      using legendre Polynomial between 0,1 for the exponential function you could get te following series
      $$e^{-frac{p}{2}}=sum _{n=0}^{infty } sqrt{frac{pi }{2}} (-1)^{ n} (2 n+1) I_{n+frac{1}{2}}(1) P_nleft(frac{p}{2}right)$$ it is suppose that t is a good aproximation using only m=2 you get the error



      $$e^{-frac{p}{2}}simeq -frac{5}{8} sqrt{frac{pi }{2}} left(3 p^2-4right) I_{frac{5}{2}}(1)+frac{3}{2} sqrt{frac{pi }{2}} p I_{frac{3}{2}}(1)+left(-sqrt{frac{pi }{2}}right) I_{frac{1}{2}}(1)$$ for p=1 the error it is about
      0.027875
      it is suppose that this series could be improve ( maybe get it better improve using chevyshef series) but de following series
      $$e^{-frac{p}{2}}simeq frac{3 pi (4 p+7) I_{frac{3}{4}}left(frac{p}{2}right)}{8 p^{3/4} Gamma left(frac{1}{4}right)}-frac{15 pi sqrt[4]{p} I_{-frac{1}{4}}left(frac{p}{2}right)}{8 Gamma left(frac{1}{4}right)}$$
      for m=2 (usin two terms) gives te error -0.00286246 10 times better
      how it is possible? it is suppose that Legendre it is orthogonal polinomial










      share|cite|improve this question







      New contributor




      CLERKRAMA is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      using legendre Polynomial between 0,1 for the exponential function you could get te following series
      $$e^{-frac{p}{2}}=sum _{n=0}^{infty } sqrt{frac{pi }{2}} (-1)^{ n} (2 n+1) I_{n+frac{1}{2}}(1) P_nleft(frac{p}{2}right)$$ it is suppose that t is a good aproximation using only m=2 you get the error



      $$e^{-frac{p}{2}}simeq -frac{5}{8} sqrt{frac{pi }{2}} left(3 p^2-4right) I_{frac{5}{2}}(1)+frac{3}{2} sqrt{frac{pi }{2}} p I_{frac{3}{2}}(1)+left(-sqrt{frac{pi }{2}}right) I_{frac{1}{2}}(1)$$ for p=1 the error it is about
      0.027875
      it is suppose that this series could be improve ( maybe get it better improve using chevyshef series) but de following series
      $$e^{-frac{p}{2}}simeq frac{3 pi (4 p+7) I_{frac{3}{4}}left(frac{p}{2}right)}{8 p^{3/4} Gamma left(frac{1}{4}right)}-frac{15 pi sqrt[4]{p} I_{-frac{1}{4}}left(frac{p}{2}right)}{8 Gamma left(frac{1}{4}right)}$$
      for m=2 (usin two terms) gives te error -0.00286246 10 times better
      how it is possible? it is suppose that Legendre it is orthogonal polinomial







      calculus






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