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Odd squarefree semiprimes n for which (n - d1)/(d1^2 - d1) = (d2^2 - d2)/(n - d2) = k, where d1 and d2 are the two prime factors of n and k is a natural number.

Duplicate of A177516.

15, 21, 33, 39, 51, 57, 65, 69, 85, 87, 91, 93, 111, 123, 129, 133, 141, 145, 159, 177, 183, 185, 201, 205, 213, 217, 219, 237, 249, 259, 265, 267, 291, 301, 303, 305, 309, 321, 327, 339, 341, 365, 381, 393, 411, 417, 427, 445, 447, 451, 453, 469, 471, 481, 485, 489, 501, 505, 511, 515, 535, 545, 553, 565, 635

All these numbers n are Fermat pseudoprimes (as odd squarefree semiprimes) for at least two bases 1 < b < n - 1.

It is interesting that all the numbers from the sequence above are Fermat pseudoprimes to base d2, d2 - 1 and d2 + 1.

Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/FermatPseudoprime.html">Fermat Pseudoprime</a>

Cf. A046388, A175101, A181780A177516.

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Marius Coman, Jul 02 2012

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All these numbers N n are Fermat pseudoprimes (as odd squarefree semiprimes) for at least two bases 1 < b < N n - 1.

Interesting It is interesting that all the numbers from the sequence above are Fermat pseudoprimes to base d2, d2 - 1 and d2 + 1.

Cf. A181780, A046388, A175101, A181780.

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15, 21, 33, 39, 51, 57, 65, 69, 85, 87, 91, 93, 111, 123, 129, 133, 141, 145, 159, 177, 183, 185, 201, 217, 205, 213, 217, 219, 237, 249, 259, 265, 267, 291, 301, 303, 305, 309, 321, 327, 339, 341, 365, 381, 393, 411, 417, 427, 445, 447, 451, 453, 469, 471, 481, 485, 489, 501, 505, 511, 515, 535, 545, 553, 565, 635

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Cf. A181780, A146166, A046388, A175101.

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Odd squarefree semiprimes N n for which (N n - d1)/(d1^2 - d1) = (d2^2 - d2)/(N n - d2) = n, k, where d1 and d2 are the two prime factors of N n and n k is a natural number.

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