A379372 Sphenic numbers k such that 2*k+1 is also a sphenic number.

322, 357, 370, 402, 430, 442, 610, 654, 790, 822, 826, 874, 885, 942, 1045, 1054, 1105, 1130, 1182, 1222, 1342, 1358, 1414, 1510, 1578, 1582, 1677, 1702, 1738, 1742, 1767, 1798, 1802, 1810, 1842, 1947, 2014, 2035, 2086, 2185, 2222, 2247, 2282, 2334, 2365, 2397

Offset: 1

Author

Paul Duckett, Dec 21 2024

Keywords

• nonn
• easy

Examples

			322 is a term because 322 = 2*7*23 (sphenic) and 2*322+1 = 645 = 3*5*43 (sphenic).
		

Crossrefs

Cf. A007304, A177221.

Programs

Mathematica

sphenicQ[n_] := FactorInteger[n][[;; , 2]] == {1, 1, 1}; Select[Range[3000], And @@ sphenicQ /@ {#, 2*# + 1} &] (* Amiram Eldar, Dec 21 2024 *)