Project Euler/28

From charlesreid1

Spiral Diagonals Sum Analysis

This page provides a mathematical analysis and algorithm to compute the sum of numbers on the diagonals of an spiral grid, starting from 1 at the center and moving clockwise. The problem is to find the sum for a spiral, given that the sum for a spiral is 101.

Understanding the 5×5 Spiral

Consider the spiral provided as an example: 

21 22 23 24 25
20  7  8  9 10
19  6  1  2 11
18  5  4  3 12
17 16 15 14 13

The diagonals are:

  • Main diagonal (top-left to bottom-right): 21, 7, 1, 3, 13.
  • Anti-diagonal (top-right to bottom-left): 25, 9, 1, 5, 17.

The sum of these numbers is:

  • Main diagonal:
  • Anti-diagonal:

Total: . Since the center (1) is counted twice, subtract it once: , which matches the given sum.

Analyzing the Spiral Pattern

The spiral starts at 1 in the center and moves outward in layers, clockwise. For a grid:

  • Layer 0: Center (1).
  • Layer 1: grid (numbers 2 to 9).
  • Layer 2: grid (numbers 10 to 25).

For an Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle n \times n} grid where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle n} is odd (), the number of layers Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle k} is:

  • Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle n = 5} , so Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 5 = 2k + 1} , Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle k = 2} .
  • Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle n = 1001} , so Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 1001 = 2k + 1} , Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle k = 500} .

Each layer Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle m} forms a square of side length Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 2m + 1} . The numbers on the diagonals are the corners of these layers.

Number of Elements per Layer

  • Layer 0: 1 number.
  • Layer 1: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 3 \times 3 = 9} numbers, minus the center, so 8 numbers.
  • Layer 2: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 5 \times 5 = 25} numbers, minus the inner Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 3 \times 3} (9 numbers), so Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 25 - 9 = 16} numbers.

For layer Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle m} , the side length is Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 2m + 1} , so the grid has Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle (2m + 1)^2} numbers. The inner grid (layer Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle m-1} ) has Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle (2m - 1)^2} numbers. The numbers in layer Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle m} :

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle (2m + 1)^2 - (2m - 1)^2 = (4m^2 + 4m + 1) - (4m^2 - 4m + 1) = 8m}

Total numbers up to layer Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle m} :

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 1 + \sum_{i=1}^m 8i = 1 + 8 \cdot \frac{m(m+1)}{2} = 1 + 4m(m+1)}

The last number in layer Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle m} (top-right corner) is Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle (2m + 1)^2} .

Corner Numbers on Diagonals

For layer Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle m} , the corners are:

  • Top-right: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle (2m + 1)^2}
  • Top-left: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle (2m + 1)^2 - 2m}
  • Bottom-left: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle (2m + 1)^2 - 4m}
  • Bottom-right: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle (2m + 1)^2 - 6m}

Verify for Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 5 \times 5} (Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle k = 2} ):

  • Layer 1 (Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle m = 1} ):
    • Top-right: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle (2 \times 1 + 1)^2 = 9}
    • Top-left: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 9 - 2 \times 1 = 7}
    • Bottom-left: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 9 - 4 \times 1 = 5}
    • Bottom-right: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 9 - 6 \times 1 = 3}
  • Layer 2 (Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle m = 2} ):
    • Top-right: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle (2 \times 2 + 1)^2 = 25}
    • Top-left: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 25 - 2 \times 2 = 21}
    • Bottom-left: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 25 - 4 \times 2 = 17}
    • Bottom-right: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 25 - 6 \times 2 = 13}

These match the grid.

Sum of Diagonals for 1001×1001 Spiral

For Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle n = 1001} , Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle k = 500} . Sum the corners from layer 1 to 500, plus the center (1).

Sum of corners in layer Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle m} :

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 4(2m + 1)^2 - (2m + 4m + 6m) = 16m^2 + 4m + 4}

Total sum from layer 1 to 500:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \sum_{m=1}^{500} (16m^2 + 4m + 4) = 16 \sum_{m=1}^{500} m^2 + 4 \sum_{m=1}^{500} m + 4 \sum_{m=1}^{500} 1}

Using:

  • Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \sum_{m=1}^n m^2 = \frac{n(n+1)(2n+1)}{6}}
  • Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \sum_{m=1}^n m = \frac{n(n+1)}{2}}
  • Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \sum_{m=1}^n 1 = n}

For Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle n = 500} :

  • Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \sum_{m=1}^{500} m^2 = \frac{500 \cdot 501 \cdot 1001}{6} = 41791750}
  • Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \sum_{m=1}^{500} m = \frac{500 \cdot 501}{2} = 125250}
  • Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \sum_{m=1}^{500} 1 = 500}

Total:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 16 \cdot 41791750 + 4 \cdot 125250 + 4 \cdot 500 = 669171000}

Add the center: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 669171000 + 1 = \mbox{the final answer}} .

Algorithm to Compute the Sum

Below is a pseudocode algorithm to compute the sum efficiently: 

function sumDiagonals(n):
    k = (n - 1) / 2
    sum = 1  # Center
    for m from 1 to k:
        sum += 16 * m * m + 4 * m + 4
    return sum

n = 1001
result = sumDiagonals(n)
print(result)

This algorithm avoids generating the entire spiral, focusing on the diagonal numbers.

Final Answer

The sum of the numbers on the diagonals of a Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle 1001 \times 1001} spiral is


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Grid 0: Problems 1-99

Problem 1  · Problem 2  · Problem 3  · Problem 4  · Problem 5  · Problem 6  · Problem 7  · Problem 8  · Problem 9  · Problem 10

Problem 11  · Problem 12  · Problem 13  · Problem 14  · Problem 15  · Problem 16  · Problem 17  · Problem 18  · Problem 19  · Problem 20

Problem 21  · Problem 22  · Problem 23  · Problem 24  · Problem 25  · Problem 26  · Problem 27  · Problem 28  · Problem 29  · Problem 30

Problem 31  · Problem 32  · Problem 33  · Problem 34  · Problem 35  · Problem 36  · Problem 37  · Problem 38  · Problem 39  · Problem 40

Problem 41  · Problem 42  · Problem 43  · Problem 44  · Problem 45  · Problem 46  · Problem 47  · Problem 48  · Problem 49  · Problem 50

Problem 51  · Problem 52  · Problem 53  · Problem 54  · Problem 55  · Problem 56  · Problem 57  · Problem 58  · Problem 59  · Problem 60

Problem 61  · Problem 62  · Problem 63  · Problem 64  · Problem 65  · Problem 66  · Problem 67  · Problem 68  · Problem 69  · Problem 70

Problem 71  · Problem 72  · Problem 73  · Problem 74  · Problem 75  · Problem 76  · Problem 77  · Problem 78  · Problem 79  · Problem 80

Grid 1: Problems 100-199

Problem 100  · Problem 101  · Problem 102  · Problem 103  · Problem 104  · Problem 105  · Problem 106  · Problem 107  · Problem 108  · Problem 109

...

Problem 150  · Problem 151  · Problem 152  · Problem 153  · Problem 154  · Problem 155  · Problem 156  · Problem 157  · Problem 158  · Problem 159

...

Problem 170  · Problem 171  · Problem 172  · Problem 173  · Problem 174  · Problem 175  · Problem 176  · Problem 177  · Problem 178  · Problem 179

...


Problem 190  · Problem 191  · Problem 192  · Problem 193  · Problem 194  · Problem 195  · Problem 196  · Problem 197  · Problem 198  · Problem 199


Grid 2: Problems 200-299

Problem 227  · Problem 254


Grid 3: Problems 300-399


Problem 301  · Problem 310

Problem 312  · Problem 319

Problem 323  · Problem 328

Problem 334  · Problem 337

Problem 345  · Problem 346

Problem 355  · Problem 356

Problem 364  · Problem 367

Problem 373  · Problem 378

Problem 382  · Problem 389

Problem 391

Grid 5: Problems 500-599

Problem 400


Grid 5: Problems 500-599

Problem 500  · Problem 501  · Problem 502


Grid 9: Problems 900-999

Problem 932


* = in progress

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