A190275 Semiprimes of the form p*(p^2 - p + 1).

6, 21, 301, 2041, 296341, 486877, 2666437, 3420301, 4304341, 7152001, 38159521, 42387097, 54296677, 95235601, 158048281, 229971241, 265434901, 383712781, 454166017, 775307917, 972261181, 1063290841, 1304557801, 1392422041, 1730882401, 1863895261, 2631883561, 2879450461, 3714274297, 3845297341, 4070454361, 4256780041, 4849695001, 5328809461, 5722533337, 5838483601, 7218898681, 7841065621

Offset

1,1

Comments

This sequence is infinite, assuming Schinzel's Hypothesis H.

Related to Rassias Conjecture ("for any odd prime p there are primes q < r such that p*q = q + r + 1") setting p = q. Generalization can be achieved by removing semiprimality condition and accepting p^e, e >= 2.

These are semiprimes m = p*q such that 1/p + 1/q - 1/m = p/q. Cf. A326690. - Amiram Eldar and Thomas Ordowski, Jul 22 2019

Links

Giovanni Resta, Table of n, a(n) for n = 1..10000

For Rassias conjecture: Preda Mihăilescu, Review of Problem Solving and Selected Topics in Number Theory, Newsletter of the European Mathematical Society, March 2011, p. 46.

Example

a(1) = 6 = 2*3 = 2*(2^2-2+1).
a(2) = 21 = 3*7 = 3*(3^2-3+1).
a(3) = 301 = 7*43 = 7*(7^2-7+1).

Maple

seq(`if`(isprime((ithprime(i)^2-ithprime(i)+1))=true, (ithprime(i)^2-ithprime(i)+1)*ithprime(i), NULL), i=1..300);

Mathematica

p = Select[Prime@ Range@ 500, PrimeQ[#^2 - # + 1] &]; p (p^2 - p + 1) (* Giovanni Resta, Jul 22 2019 *)

Prog

(PARI) forprime(p=2, 1e4, if(isprime(k=p^2-p+1), print1(p*k", "))) \\ Charles R Greathouse IV, May 08 2011

Crossrefs

Cf. A065508 (primes p such that p^2-p+1 is prime).

Cf. A001358 (semiprime), A003415 (arithmetic derivative), A164643, A190272 (n'=a-1), A190273 (n'=a+1), A190274 (n'=p^2-1).

Sequence in context: A143049 A213680 A164643 * A261844 A372425 A007594

Adjacent sequences: A190272 A190273 A190274 * A190276 A190277 A190278

Keyword

nonn

Author

Giorgio Balzarotti, May 07 2011

Status

approved