A267797 Lucas numbers of the form (x^3 + y^3) / 2 where x and y are distinct positive integers.

76, 1364, 24476, 439204, 7881196, 141422324, 2537720636, 45537549124, 817138163596, 14662949395604, 263115950957276, 4721424167835364, 84722519070079276, 1520283919093591604, 27280388024614569596, 489526700523968661124, 8784200221406821330636

Offset

1,1

Comments

Lucas numbers that are the averages of 2 distinct positive cubes.

Inspired by relation between sequence A024851 and A188378.

Corresponding indices are 9, 15, 21, 27, 33, 39, 45, 51, 57, 63, 69, 75, 81, 87, 93, 99, 105, 111, 117, ...

6*n + 3 is the corresponding form of indices.

Corresponding y values are listed by A188378, for n > 0. Note that corresponding x values are A188378(n) - 2, for n > 0.

Links

Colin Barker, Table of n, a(n) for n = 1..750

Index entries for linear recurrences with constant coefficients, signature (18,-1).

Formula

a(n) = A000032(A016945(n)), for n > 0.

a(n) = A188378(n)^3 - 3*A188378(n)^2 + 6*A188378(n) - 4, for n > 0.

From Colin Barker, Jan 24 2016: (Start)

a(n) = (9+4*sqrt(5))^(-n)*(2-sqrt(5)+(2+sqrt(5))*(9+4*sqrt(5))^(2*n)).

a(n) = 18*a(n-1)-a(n-2) for n>2.

G.f.: 4*x*(19-x) / (1-18*x+x^2).

(End)

Example

Lucas number 76 is a term because 76 = (3^3 + 5^3) / 2.
Lucas number 1364 is a term because 1364 = (10^3 + 12^3) / 2.
Lucas number 24476 is a term because 24476 = (28^3 + 30^3) / 2.
Lucas number 439204 is a term because 439204 = (75^3 + 77^3) / 2.
Lucas number 7881196 is a term because 7881196 = (198^3 + 200^3) / 2.
Lucas number 141422324 is a term because 141422324 = (520^3 + 522^3) / 2.

Mathematica

Table[Fibonacci[6 n + 4] + Fibonacci[6 n + 2], {n, 1, 20}] (* Vincenzo Librandi, Jan 24 2016 *)

Prog

(PARI) l(n) = fibonacci(n+1) + fibonacci(n-1);
is(n) = for(i=ceil(sqrtn(n\2+1, 3)), sqrtn(n-.5, 3), ispower(n-i^3, 3) && return(1));
for(n=1, 120, if(is(2*l(n)), print1(l(n), ", ")));
(PARI) a(n) = ((5*fibonacci(n)*fibonacci(n+1) + 1 + (-1)^n)^3 + (5*fibonacci(n)*fibonacci(n+1) - 1 + (-1)^n)^3) / 2;
(PARI) a(n) = (fibonacci(6*n+4) + fibonacci(6*n+2));
(PARI) Vec(4*x*(19-x)/(1-18*x+x^2) + O(x^20)) \\ Colin Barker, Jan 24 2016
(Magma) [Fibonacci(6*n+4)+Fibonacci(6*n+2): n in [1..20]]; // Vincenzo Librandi, Jan 24 2016

Crossrefs

Cf. A000032, A016945, A024670, A024851, A049629, A188378.

Sequence in context: A233365 A264627 A136539 * A163710 A293310 A061618

Adjacent sequences: A267794 A267795 A267796 * A267798 A267799 A267800

Keyword

nonn,easy

Author

Altug Alkan, Jan 24 2016

Status

approved