Input
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What does the following represent: ex2dx=π\int_{-\infty}^\infty e^{-x^2} \,dx = \sqrt{\pi}


The expression ex2dx=π\int_{-\infty}^\infty e^{-x^2} \,dx = \sqrt{\pi} represents the integral of the Gaussian function ex2e^{-x^2} over the entire real line. The result of this integral, π\sqrt{\pi}, emerges from a deep connection between the exponential function and the geometry of the real number line, specifically related to the normal distribution in statistics. This integral is vitally important in probability theory, statistics, and various fields of physics and engineering. Here's how this result can be understood and derived:

Understanding ex2e^{-x^2}

The function ex2e^{-x^2} rapidly approaches zero as xx moves away from zero (either positively or negatively), more quickly than any power of 1/x1/x. It's a bell-shaped curve, symmetrical about the y-axis, known as the Gaussian function or bell curve.

The value of ex2dx=π\int_{-\infty}^\infty e^{-x^2} \,dx = \sqrt{\pi}

This integral, which evaluates the entire area under the Gaussian curve, is famously computed using a clever double integration technique involving polar coordinates:

  1. Square the Integral: Consider I=ex2dxI = \int_{-\infty}^\infty e^{-x^2} \,dx. If we square this integral, we obtain: [ I^2 = \left(\int_{-\infty}^\infty e^{-x^2} ,dx \right) \left(\int_{-\infty}^\infty e^{-y^2} ,dy \right) = \int_{-\infty}^\infty \int_{-\infty}^\infty e^{-x^2}e^{-y^2} ,dy,dx. ] We can factorize the exponent in the integrand: [ I^2 = \int_{-\infty}^\infty \int_{-\infty}^\infty e^{-(x^2 + y^2)} ,dx,dy. ]

  2. Change to Polar Coordinates: By converting the Cartesian coordinates (x,y)(x, y) to polar coordinates (r,θ)(r, \theta), where x=rcosθx = r\cos\theta and y=rsinθy = r\sin\theta, the expression x2+y2=r2x^2 + y^2 = r^2 simplifies the double integral, which becomes: [ I^2 = \int_0^{2\pi} \int_0^\infty e^{-r^2} r ,dr,d\theta. ] Through a substitution of u=r2u = r^2, du=2rdrdu = 2r \,dr, this turns into: [ I^2 = \int_0^{2\pi} \int_0^\infty e^{-u} \frac{1}{2} ,du,d\theta = \frac{1}{2}\int_0^{2\pi} ,d\theta = \pi, ] where the inner integral solves to 1/21/2 as integrating eue^{-u} from 0 to \infty yields 1.

  3. Final Computation: Thus, I2=πI^2 = \pi, and taking the square root on both sides gives I=πI = \sqrt{\pi}.

This computation not only has theoretical importance but also applications in fields where normal distributions or related notions of smoothing and decay are utilized.


Evaluation

Evaluation ID
c3b885bf-1ee0-454b-9bb3-c39dcc245fdb
Created
April 23, 2024

Evaluation Rating

Binary Pass/Fail
Pass
Score
100
Additional Feedback
The submission provides a comprehensive and accurate explanation of the Gaussian integral and its significance. It goes into detail about the exponential function, the geometry behind the integral, the double integration technique involving polar coordinates, and the final computation. The connection to probability theory, statistics, and various fields of physics and engineering is highlighted effectively.
Factual Consistency
B: the submitted answer is a superset of the expert answer and is fully consistent with it
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        "content": "The expression $\\int_{-\\infty}^\\infty e^{-x^2} \\,dx = \\sqrt{\\pi}$ represents the integral of the Gaussian function $e^{-x^2}$ over the entire real line. The result of this integral, $\\sqrt{\\pi}$, emerges from a deep connection between the exponential function and the geometry of the real number line, specifically related to the normal distribution in statistics. This integral is vitally important in probability theory, statistics, and various fields of physics and engineering. Here's how this result can be understood and derived:\n\n### Understanding $e^{-x^2}$\nThe function $e^{-x^2}$ rapidly approaches zero as $x$ moves away from zero (either positively or negatively), more quickly than any power of $1/x$. It's a bell-shaped curve, symmetrical about the y-axis, known as the Gaussian function or bell curve.\n\n### The value of $\\int_{-\\infty}^\\infty e^{-x^2} \\,dx = \\sqrt{\\pi}$\nThis integral, which evaluates the entire area under the Gaussian curve, is famously computed using a clever double integration technique involving polar coordinates:\n\n1. **Square the Integral:**\n   Consider $I = \\int_{-\\infty}^\\infty e^{-x^2} \\,dx$. If we square this integral, we obtain:\n   \\[\n   I^2 = \\left(\\int_{-\\infty}^\\infty e^{-x^2} \\,dx \\right) \\left(\\int_{-\\infty}^\\infty e^{-y^2} \\,dy \\right) = \\int_{-\\infty}^\\infty \\int_{-\\infty}^\\infty e^{-x^2}e^{-y^2} \\,dy\\,dx.\n   \\]\n   We can factorize the exponent in the integrand:\n   \\[\n   I^2 = \\int_{-\\infty}^\\infty \\int_{-\\infty}^\\infty e^{-(x^2 + y^2)} \\,dx\\,dy.\n   \\]\n\n2. **Change to Polar Coordinates:**\n   By converting the Cartesian coordinates $(x, y)$ to polar coordinates $(r, \\theta)$, where $x = r\\cos\\theta$ and $y = r\\sin\\theta$, the expression $x^2 + y^2 = r^2$ simplifies the double integral, which becomes:\n   \\[\n   I^2 = \\int_0^{2\\pi} \\int_0^\\infty e^{-r^2} r \\,dr\\,d\\theta.\n   \\]\n   Through a substitution of $u = r^2$, $du = 2r \\,dr$, this turns into:\n   \\[\n   I^2 = \\int_0^{2\\pi} \\int_0^\\infty e^{-u} \\frac{1}{2} \\,du\\,d\\theta = \\frac{1}{2}\\int_0^{2\\pi} \\,d\\theta = \\pi,\n   \\]\n   where the inner integral solves to $1/2$ as integrating $e^{-u}$ from 0 to $\\infty$ yields 1.\n\n3. **Final Computation:**\n   Thus, $I^2 = \\pi$, and taking the square root on both sides gives $I = \\sqrt{\\pi}$.\n\nThis computation not only has theoretical importance but also applications in fields where normal distributions or related notions of smoothing and decay are utilized."
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