Counting Shortest Paths in a Square Grid

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Discussion Overview

The discussion revolves around counting the number of shortest paths in a square grid, specifically from the lower left corner to the upper right corner, while also considering partial paths to any intersection or corner within the grid. The context includes both theoretical and mathematical reasoning related to combinatorial paths in a grid structure.

Discussion Character

  • Mathematical reasoning
  • Exploratory

Main Points Raised

  • One participant asks for a concise method to count all paths from the lower left to the upper right corner of a grid, including partial paths to other intersections.
  • Another participant suggests that there is a straightforward solution for reaching any point on the grid, referencing the combinatorial formula for paths as (m+n) choose n.
  • A different participant acknowledges the challenge of summing all paths concisely, indicating that this is a more complex problem than simply counting paths to a single destination.
  • One participant proposes that summing paths in a specific triangular region of the grid can be done easily, relating it to powers of 2 and Pascal's triangle, providing a specific numerical result for that case.

Areas of Agreement / Disagreement

Participants express differing views on the complexity of summing all paths, with some suggesting it is straightforward while others indicate it is more challenging. The discussion remains unresolved regarding the best approach to count all paths.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the grid structure and the specific definitions of "partial paths." The mathematical steps for summing paths are not fully resolved.

Bartholomew
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Is there a concise, easily calculable way to count the sum of all the paths from the lower left corner of a square of integer size to the upper right corner where you only move up or right in steps of 1 unit, PLUS all the partial paths? i.e. if you have a city with roads in a 10x10 grid (100 intersections/corners) how many shortest paths are there from one corner of the grid to any other intersection or corner on the grid?
 
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Bartholomew said:
Is there a concise, easily calculable way to count the sum of all the paths from the lower left corner of a square of integer size to the upper right corner where you only move up or right in steps of 1 unit, PLUS all the partial paths? i.e. if you have a city with roads in a 10x10 grid (100 intersections/corners) how many shortest paths are there from one corner of the grid to any other intersection or corner on the grid?

Why stop there? There's an easy solution for any point on a grid like that.


To get to the point m,n from 0,0 there are (m+n) choose n paths.
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I know that. Adding them all up concisely is a harder problem.
 
It's easy to sum the paths in the triangle bounded by (and including) the line y = x + 10--that's 2^10 - 1 since each diagonal row of intersections/corners (with negative slope) in that triangle corresponds to a row of Pascal's triangle, so you get 2^0 + 2^1 + ... + 2^9 which is 1111111111 in binary.
 
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