diff --git a/notebooks/problem0053.ipynb b/notebooks/problem0053.ipynb new file mode 100644 index 0000000..b3e6fb4 --- /dev/null +++ b/notebooks/problem0053.ipynb @@ -0,0 +1,78 @@ +{ + "cells": [ + { + "cell_type": "markdown", + "id": "59fa6654", + "metadata": {}, + "source": [ + "# [Combinatoric Selections](https://projecteuler.net/problem=53)\n", + "\n", + "Python/SageMath's [unlimited precision integers](https://docs.python.org/3/library/stdtypes.html#numeric-types-int-float-complex) make this trivial." + ] + }, + { + "cell_type": "code", + "execution_count": 1, + "id": "abf93cda", + "metadata": {}, + "outputs": [ + { + "data": { + "text/plain": [ + "4075" + ] + }, + "execution_count": 1, + "metadata": {}, + "output_type": "execute_result" + } + ], + "source": [ + "binoms = (binomial(n, r) for n in range(1, 101) for r in range(0, n + 1))\n", + "len([b for b in binoms if b > 1000000])" + ] + }, + { + "cell_type": "markdown", + "id": "6d7f4409", + "metadata": {}, + "source": [ + "If it weren't for unlimited precision, we would have a problem, since a lot of these [binomial coefficients](https://en.wikipedia.org/wiki/Binomial_coefficient) overflow a 64 bit integer - the largest one we encounter here is $\\binom{100}{50} = 100891344545564193334812497256$.\n", + "\n", + "However, even in that case there's a pretty simple workaround: finding $n$ and $r$ such that\n", + "$$\\binom{n}{r} > 1000000$$\n", + "is the same as finding $n$ and $r$ such that\n", + "$$\\log{\\binom{n}{r}} > \\log{1000000}$$\n", + "By applying the definition of the binomial coefficients and [logarithmic identities](https://en.wikipedia.org/wiki/List_of_logarithmic_identities), this turns to\n", + "$$\\log{n!} - \\log{r!} - \\log{(n-r)!} > \\log{1000000}$$\n", + "$\\log{100!} \\approx 363.739$, nowhere close to overflowing a float.\n", + "\n", + "$\\log{n!}$ can be computed in a number of ways. You can use the [log-gamma function](https://en.wikipedia.org/wiki/Gamma_function), or you can implement a function yourself, either by applying logarithmic identities again:\n", + "$$\\log{n!} = \\log{1} + \\log{2} + \\log{3} \\cdots + \\log{n}$$\n", + "or by using [Stirling's approximation](https://en.wikipedia.org/wiki/Stirling%27s_approximation):\n", + "$$\\log{n!} \\approx \\left(n + \\frac{1}{2}\\right)\\log{n} - n + \\frac{1}{2}\\log{2\\pi}$$" + ] + } + ], + "metadata": { + "kernelspec": { + "display_name": "SageMath 9.5", + "language": "sage", + "name": "sagemath" + }, + "language_info": { + "codemirror_mode": { + "name": "ipython", + "version": 3 + }, + "file_extension": ".py", + "mimetype": "text/x-python", + "name": "python", + "nbconvert_exporter": "python", + "pygments_lexer": "ipython3", + "version": "3.11.2" + } + }, + "nbformat": 4, + "nbformat_minor": 5 +}