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PROJECT EULER · #0229

Four Representations Using Squares

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Consider the number 3600. It is very special, because

3600=482+3623600=202+2×4023600=302+3×3023600=452+7×152

Similarly, we find that 88201=992+2802=2872+2×542=2832+3×522=1972+7×842.

In 1747, Euler proved which numbers are representable as a sum of two squares. We are interested in the numbers n which admit representations of all of the following four types:

n=a12+b12n=a22+2b22n=a32+3b32n=a72+7b72,

where the ak and bk are positive integers.

There are 75373 such numbers that do not exceed 107.
How many such numbers are there that do not exceed 2×109?

Write-up coming later

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