A334900 Numbers k such that k and k+2 are both bi-unitary practical numbers (A334898).

6, 30, 40, 54, 510, 544, 798, 918, 928, 1120, 1240, 1288, 1408, 1480, 1566, 1672, 1720, 1768, 1792, 1888, 1950, 1974, 2046, 2430, 2440, 2560, 2728, 2814, 2838, 2968, 3198, 3318, 4134, 4158, 4264, 4422, 4480, 4758, 5248, 6102, 6270, 6424, 6942, 7590, 7830, 9280

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..112

Example

6 is a term since 6 and 6 + 2 = 8 are both bi-unitary practical numbers.

Mathematica

biunitaryDivisorQ[div_, n_] := If[Mod[#2, #1] == 0, Last@Apply[Intersection, Map[Select[Divisors[#], Function[d, CoprimeQ[d, #/d]]] &, {#1, #2/#1}]] == 1, False] & @@ {div, n}; bdivs[n_] := Module[{d = Divisors[n]}, Select[d, biunitaryDivisorQ[#, n] &]]; bPracQ[n_] := Module[{d = bdivs[n], sd, x}, sd = Plus @@ d; Min @ CoefficientList[Series[Product[1 + x^d[[i]], {i, Length[d]}], {x, 0, sd}], x] >  0]; seq = {}; q1 = bPracQ[2]; Do[q2 = bPracQ[n]; If[q1 && q2, AppendTo[seq, n - 2]]; q1 = q2, {n, 4, 1000, 2}]; seq

Crossrefs

Cf. A287681, A330871, A334882, A334898, A334903.

Sequence in context: A345265 A062268 A067879 * A136375 A138706 A027762

Adjacent sequences: A334897 A334898 A334899 * A334901 A334902 A334903

Keyword

nonn

Author

Amiram Eldar, May 16 2020

Status

approved