The integration shown here, ∫+∞−∞xrExp[−x2]Hn2[x]dx, appears when we try to calculate the spectrum of the perturbed non-linear oscillators by using perturbation theory in quantum mechanics. Is there any direct way to perform the definite integration of the form shown above. I want the solution of the integral for r≥4. I hope there may exist some techniques which can be used to calculate the integration of above integral. Please i need suggestion from this forum to solve this integration. Highly appreciated!
Answer
Note : Below a generating function is derived for calculating the following matrix element (useful in perturbative computations) : Om,n,k=2k√π∫+∞−∞dxHm(x)xkHn(x)e−x2. where {Hr(x)} are hermite polynamials and k is a non-negative integer.
(i) Consider the generating function of the Hermite polynomials :
G[z,x]=∞∑n=0znn!Hn(x)=e2xz−z2.
(ii) Define a generating function Z[z1,z2,z3] :
Z[z1,z2,z3]=1√π∫+∞−∞dxe−x2G[z1,x]G[z2,x]e2xz3=e−[z21+z22]√π∫+∞−∞dxe−x2+2x[z1+z2+z3] ⇒Z[z1,z2,z3]=e−[z21+z22]+[z1+z2+z3]2. (iii) Now notice : Om,n,k=(∂∂z1)m(∂∂z2)n(∂∂z3)kZ[z1,z2,z3]|(z1,z2,z3)=(0,0,0). (iv) Hence : Om,n,k=(∂∂z1)m(∂∂z2)n(∂∂z3)ke−[z21+z22]+[z1+z2+z3]2|(z1,z2,z3)=(0,0,0) ∴
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