This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A268043 Numbers k such that k^3 - 1 and k^3 + 1 are semiprime. 5
 6, 1092, 1932, 2730, 4158, 6552, 11172, 25998, 30492, 55440, 76650, 79632, 85092, 102102, 150990, 152082, 152418, 166782, 211218, 235662, 236208, 248640, 264600, 298410, 300300, 301182, 317772, 380310, 387198, 441798, 476028, 488418 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Obviously, k+1 and k-1 are always prime numbers. LINKS EXAMPLE a(1) = 6 because 6^3-1 = 215 = 5*43 and 6^3+1 = 217 = 7*31, therefore 215, 217 are both semiprimes. MATHEMATICA Select[Range[500000], PrimeOmega[#^3 - 1] == PrimeOmega[#^3 + 1] == 2 &] PROG (MAGMA) IsSemiprime:=func< n | &+[k[2]: k in Factorization(n)] eq 2 >; [ n: n in [2..300000] | IsSemiprime(n^3+1) and IsSemiprime(n^3-1) ]; (PARI) isok(n) = (bigomega(n^3-1) == 2) && (bigomega(n^3+1) == 2); \\ Michel Marcus, Jan 26 2016 CROSSREFS Subsequence of A014574. Cf. A002384, A014574, A055494, A088707. A096173, A096175, A109373. Sequence in context: A125536 A003763 A179853 * A190351 A267071 A219165 Adjacent sequences:  A268040 A268041 A268042 * A268044 A268045 A268046 KEYWORD nonn AUTHOR Vincenzo Librandi, Jan 25 2016 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified October 13 16:35 EDT 2019. Contains 327967 sequences. (Running on oeis4.)