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Squarefree semiprimes that are in this sequence (35, 77, 143, 187, 209, 221, ...) are all in A259282 and they are the only semiprimes there. (See the Echi and Ghanmi paper reference for a proof.) - Elijah Beregovsky, Feb 05 2020
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Squarefree semiprimes that are in this sequence (35, 77, 143, 187, 209, 221, ...) are all in A259282 and they are the only semiprimes there. (See Echi and Ghanmi paper for proof.) - Elijah Beregovsky, Feb 05 2020
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Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Multifactorial.html">Multifactorial</a>
Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Multifactorial.html">Multifactorial</a>
15 is not in the sequence because 15 = 1*3*5 = 5!!.
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The sequence contains only primes and numbers of the form p*q, where p and q are both prime and satisfy the inequalities p > = q and p-q < q-1.
Squarefree semiprimes that are in this sequence (35, 77, 143, 187, 209, 221...) are all in A259282 and they are the only semiprimes there. (See Echi and Ghanmi paper for proof.) - Elijah Beregovsky, Feb 05 2020
O. Echi, N. Ghanmi, <a href="https://www.researchgate.net/publication/267677588_The_Korselt_set_of_pq">The Korselt set of pq</a>, International Journal of Number Theory, Vol. 8 (2012), 2, 299-309.
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