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# pjt33

Created Jan 21, 2020
Count solutions to linear Diophantine equations
View linear_diophantine.py
 from collections import Counter from fractions import Fraction def _gcd(a, b): while a: a, b = b % a, a return b
Last active Nov 23, 2019
Dragon merging
View math3444274.py
 #!/usr/bin/python3 # https://math.stackexchange.com/q/3444274 from fractions import Fraction def build_states(target_dragon): states = {} def toBinary(x, width):
Created Oct 30, 2019
Math 3414931
View math3414931.py
 #!/usr/bin/python3 from functools import lru_cache @lru_cache(None) def binom(n, k): if n < 0 or k < 0 or k > n: return 0 rv = 1 for i in range(k):
Last active Sep 13, 2019
Chebyshev pseudoprime test
View CR228847.numpy
 #!/usr/bin/python3 import timeit from numpy import mod from numpy.polynomial.polynomial import Polynomial def is_prime_naive(n): if n <= 2 or n % 2 == 0: return n == 2
Last active Sep 3, 2019
Strings with prefix imbalance
View math3343318.md

Let's define $a_{n,m}$ to be the number of words of length $n$ with a surplus of $m$ A's. E.g. with $k=2$, AAAB would contribute $1$ to $a_{4,1}$; with $k=3$ it would contribute $1$ to $a_{4,0}$.

Given a word of length $n-1$ we can always append an A to get a valid word; we can append a B to get a valid word iff the surplus is at least $k$. So if we set up the generating function $$f(x,z) = \sum_{n \ge 0} \sum_{m=0}^n a_{n,m} x^n z^m$$ we find that $$f(x, z) = 1 + xz f(x, z) + xz^{-p} \sum_{n \ge 0} \sum_{m=p}^n a_{n,m} x^n z^m \tag{1}$$ which doesn't look very promising.

However, I believe Deutsch proved that this kind of regression always gives a Riordan array, so let's assume that we can write $f(x, z)$ in the form $$f(x, z) = \sum_{m \ge 0} z^m d(x) (xh(x))^m \tag{2}$$

Useful observation: the length minus the surplus is always a multiple of $k+1$ (easily shown by induction), so $d(x)$ only has non-zero coefficients at powers of $x^{k+1}$, or $d(x) = d_k(x^{k+1})$.

Useful observation: if we ext

Created Sep 3, 2019
Drive Ya Nuts solution counter
View DriveYaNuts.cs
 using System; using System.Collections.Generic; using System.Linq; // WARNING: THIS IS A QUICK HACK, NOT PRODUCTION CODE. IT CAN PROBABLY BE OPTIMISED QUITE SIGNIFICANTLY. namespace Sandbox { // https://math.stackexchange.com/q/3306917 static class DriveYaNuts
Created Aug 26, 2019
Perfectly nonlinear involutions
View MO338802.cs
 using System; using System.Collections.Generic; using System.Linq; using System.Numerics; namespace Sandbox { class MO338802 { public static void Main()
Created Jan 14, 2019
Eratospheres in Swift
View Carpsen.swift
 func eratosthenesSieve(to n: Int) -> [Int] { guard 2 <= n else { return [] } var composites = Array(repeating: false, count: n + 1) var primes: [Int] = [] let d = Double(n) let upperBound = Int((d / _log(d)) * (1.0 + 1.2762/_log(d))) primes.reserveCapacity(upperBound) let squareRootN = Int(d.squareRoot())
Last active Dec 5, 2018
MSE 3015816
View mathse3015816.py
 #!/usr/bin/python3 import math def phi(n): # Naïve prime factorisation ps = set() m = n while m % 2 == 0:
Last active Oct 20, 2018
Number of real zeroes of iterated polynomial: x^3-2x+1
View math2962644.sage
 from sage.rings.polynomial.real_roots import * x = polygen(ZZ) def conv_p(p, n): if n == 0: return x else: return p(conv_p(p, n-1)) for n in range(1, 21): print add(map(lambda (a,b): b, real_roots(conv_p(x^3-2*x-1, n))))
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