OFFSET
1,1
COMMENTS
The recurrence for the general case lcm(n+k, n-k) is a(n) = 3*a(n-2*k)-3*a(n-4*k)+a(n-6*k) for n>6*k.
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (0,0,0,0,0,3,0,0,0,0,0,-3,0,0,0,0,0,1).
FORMULA
a(n) = 3*a(n-6)-3*a(n-12)+a(n-18) for n>18.
G.f.: x*(-10*x^19 -8*x^18 -3*x^17 -4*x^16 -5*x^15 +37*x^13 +32*x^12 +18*x^11 +32*x^10 +70*x^9 +12*x^8 +40*x^7 +8*x^6 +9*x^5 +8*x^4 +7*x^3 +5*x +4) / (-x^18 +3*x^12 -3*x^6 +1).
From Peter Bala, Feb 15 2019: (Start)
For n >= 3, a(n) = (n^2 - 9)/b(n), where (b(n)), n >= 3, is the periodic sequence [6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, ...] of period 6. a(n) is thus a quasi-polynomial in n
For n >= 4, a(n) = (n + 3)*A060789(n-3). (End)
Sum_{n>=4} 1/a(n) = 47/60. - Amiram Eldar, Aug 09 2022
Sum_{k=1..n} a(k) ~ 7*n^3/36. - Vaclav Kotesovec, Aug 09 2022
EXAMPLE
a(8) = 55 because lcm(8+3, 8-3) = lcm(11, 5) = 55.
MAPLE
MATHEMATICA
CoefficientList[Series[(-10 x^19 - 8 x^18 - 3 x^17 - 4 x^16 -5 x^15 + 37 x^13 + 32 x^12 + 18 x^11 + 32 x^10 + 70 x^9 + 12 x^8 + 40 x^7 + 8 x^6 + 9 x^5 + 8 x^4 + 7 x^3 + 5 x + 4) / (- x^18 + 3 x^12 - 3 x^6 + 1), {x, 0, 50}], x] (* Vincenzo Librandi, Nov 08 2014 *)
Table[LCM @@ (n + {-3, 3}), {n, 54}] (* Michael De Vlieger, Feb 13 2017 *)
PROG
(PARI) a(n) = lcm(n+3, n-3)
(PARI) Vec(x*(-10*x^19 -8*x^18 -3*x^17 -4*x^16 -5*x^15 +37*x^13 +32*x^12 +18*x^11 +32*x^10 +70*x^9 +12*x^8 +40*x^7 +8*x^6 +9*x^5 +8*x^4 +7*x^3 +5*x +4) / (-x^18 +3*x^12 -3*x^6 +1) + O(x^100))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Nov 07 2014
STATUS
approved