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Alguna vez ya publiqué una demostración de una proposición de Fermat, sobre la no existencia de triángulos rectángulos de lados enteros con área cuadrada, aplicando el método de descenso infinito. Ver:

El Ultimo Teorema de Fermat (4)
Fermat y el Descenso Infinito
Descenso Infinito

Fermat estaba muy orgulloso de haber inventado y aplicado ese método. No escribió muchas demostraciones de sus afirmaciones, aunque declaraba por escrito que estaba más que dispuesto a compartirlas si alguien lo requería. Pero, consciente o inconscientemente, Fermat no daba fácilmente explicación de su demostraciones. La proposición de arriba fue inspirada por una proposición de Bachet, el editor de la aritmética de Diofanto. Y de nuevo en esta demostración usa descenso infinito. Hoy leo un texto de Fermat dando alguna idea de su demostración:

If the area of a right-angled triangle were a square, there would exist two biquadrates the difference of which would be a square number. Consenquently there would exist two square numbers the sum and difference of which would both be squeares. Therefor we should have a square number which would be equal to the sum of a square and the double of another square, while the squares of which this sum is made up would themselves have a square number for their sum. But if a square is made up of a square and the double of another square, its side, as I can very easily prove, is also similarly made up of a square and the double of another square. From this we conclude that the said side is the sum of the sides about the right angle in a right-angled triangle, and that the simple square contained in the sum is the base and the double of the other square is perpendicular.

This right-angled triangle will thus be formed from two squares, the sum and differences of which will be squares. But both these squares can be shown to be smaller than the squares originally assumed to be such that both their sum and difference are squares. Thus if there exist two squares such that their sum and difference are both squares, there will also exist two other integer squares which have the same property but have a smaller sum. By the same reasoning we find a sum still smaller than that last found, and we can go on ad infinitum finding integer square numbers smaller and smaller which have the same property. This is, however, impossible because these cannot be an infinite series of numbers smaller than any given integer we please. Tha margin is too small to enable me to give the proof completely and with all detail.

Es un texto traducido del latín al inglés por Health. No encuentro en qué lugar está el original, pero sospecho que está en las anotaciones de Fermat a Diofanto. Es curioso que de nuevo acá vuelva a mencionar lo estrecho del margen, aunque su texto es bastante largo.

Pero la demostración que explica parte de una frase algo misteriosa: "If the area of a right-angled triangle were a square, there would exist two biquadrates the difference of which would be a square number" En próximo post intentaré explicar de donde parte Fermat, qué quiere indicar con esta afirmación.

Encuentro el texto citado en el excelente libro Fermat's Last Theorem, A Genetic Introduction to Algebraic Number Theory, de Harold M. Edwards.

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Angel "Java" Lopez
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