%I #78 Sep 11 2024 00:39:58
%S 1,2,4,8,16,24,36,54,81,135,225,375,625,1000,1600,2560,4096,6656,
%T 10816,17576,28561,46137,74529,120393,194481,314874,509796,825384,
%U 1336336,2161720,3496900,5656750,9150625,14807375,23961025,38773295,62742241,101515536
%N Let r and s be consecutive Fibonacci numbers. Sequence is r^4, r^3 s, r^2 s^2, and r s^3.
%C Two consecutive Fibonacci numbers are coprime. This sequence satisfies a 14th-order linear difference equation. Note that it is the fourth sequence in the sequences that begin with the Fibonacci numbers, A006498, and A006500. Subsequent sequences will have orders 22, 32, and 44. - _T. D. Noe_, Mar 05 2012
%C Also the number of subsets of the set {1,2,...,n-1} which do not contain two elements whose difference is 4. - _David Nacin_, Mar 07 2012
%H Vincenzo Librandi, <a href="/A031923/b031923.txt">Table of n, a(n) for n = 1..1000</a>
%H Michael A. Allen, <a href="https://arxiv.org/abs/2209.01377">On a Two-Parameter Family of Generalizations of Pascal's Triangle</a>, arXiv:2209.01377 [math.CO], 2022.
%H Michael A. Allen, <a href="https://arxiv.org/abs/2409.00624">Connections between Combinations Without Specified Separations and Strongly Restricted Permutations, Compositions, and Bit Strings</a>, arXiv:2409.00624 [math.CO], 2024. See p. 16.
%H M. El-Mikkawy and T. Sogabe, <a href="https://doi.org/10.1016/j.amc.2009.12.069">A new family of k-Fibonacci numbers</a>, Appl. Math. Comput. 215 (2010) 4456-4461 doi:10.1016/j.amc.2009.12.069, Table 1 k=4.
%H M. Tetiva, <a href="http://www.jstor.org/stable/10.4169/math.mag.84.4.296">Subsets that make no difference d</a>, Mathematics Magazine 84 (2011), no. 4, 300-301.
%H <a href="/index/Rec#order_14">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,0,-2,2,2,0,2,-2,-2,0,1,-1,-1).
%F a(n) = F(floor((n-1)/4) + 3)^(n-1 mod 4)*F(floor((n-1)/4) + 2)^(4 - (n-1 mod 4)) where F(n) is the n-th Fibonacci number. - _David Nacin_, Mar 07 2012
%F a(n) = a(n-1) + a(n-2) - 2*a(n-4) + 2*a(n-5) + 2*a(n-6) + 2*a(n-8) - 2*a(n-9) - 2*a(n-10) + a(n-12) - a(n-13) - a(n-14). - _David Nacin_, Mar 07 2012
%F G.f.: x*(2 + 2*x + 2*x^2 + 4*x^3 + 4*x^4 - 2*x^6 - 1*x^7 - 4*x^8 - 3*x^9 - x^10 - x^11 - 2*x^12 - x^13)/((1 - x)*(1 + x)*(1 + x^2)*(1 - x - x^2)*(1 + 3*x^4 + x^8)). - _David Nacin_, Mar 08 2012
%F a(4*k-3) = F(k+1)^4, a(4*k-2) = F(k+1)^3*F(k+2), a(4*k-1) = F(k+1)^2*F(k+2)^2, a(4*k) = F(k+1)*F(k+2)^3, k >= 1, where F = A000045. - _Jianing Song_, Feb 06 2019
%F a(4n+1)= A056571(n+2). a(4n+3)=A197424(n). - _R. J. Mathar_, Jan 23 2022
%e Since F_5 = 5 and F_6 = 8 are consecutive Fibonacci numbers, 8^4 = 4096, 8^3*5 = 2560, 8^2*5^2 = 1600, 8*5^3 = 1000, and 5^4 = 625 are in the sequence.
%e The number 3^3*8 = 216 is not in the sequence since 3 and 8 are not consecutive.
%e If n = 6 then this gives the number of subsets of {1,...,5} not containing both 1 and 5. There are 2^3 subsets containing 1 and 5, giving us 2^5 - 2^3 = 24. Thus a(5) = 24. - _David Nacin_, Mar 07 2012
%p A031923 := proc(n)
%p local n0,i,r,s,m ;
%p n0 := n-1 ;
%p i := floor(n0/4) ;
%p r := combinat[fibonacci](i+2) ;
%p s := combinat[fibonacci](i+3) ;
%p m := modp(n0,4) ;
%p r^(4-m)*s^m ;
%p end proc:
%p seq(A031923(n),n=1..50) ; # _R. J. Mathar_, Jan 23 2022
%t f = Fibonacci[Range[12]]; m = Most[f]; r = Rest[f]; Union[m^4, m^3 r, m^2 r^2, m r^3] (* _T. D. Noe_, Mar 05 2012 *)
%t LinearRecurrence[{1, 1, 0, -2, 2, 2, 0, 2, -2, -2, 0, 1, -1, -1}, {1, 2, 4, 8, 16, 24, 36, 54, 81, 135, 225, 375, 625, 1000}, 40] (* _T. D. Noe_, Mar 05 2012 *)
%t Table[Fibonacci[Floor[n/4] + 3]^Mod[n, 4]*Fibonacci[Floor[n/4] + 2]^(4 - Mod[n, 4]), {n, 0, 40}] (* _David Nacin_, Mar 07 2012 *)
%t cfn[{a_,b_}]:={a^4,a^3 b,a^2 b^2,a b^3}; Flatten[cfn/@Partition[ Fibonacci[ Range[20]],2,1]]//Union (* _Harvey P. Dale_, Feb 03 2019 *)
%o (PARI) for(m=2,10,r=fibonacci(m);s=fibonacci(m+1);print(r^4," ",r^3*s," ",r^2*s^2," ",r*s^3)) \\ _Michael B. Porter_, Mar 04 2012
%o (Python)
%o def a(n, adict={0:0, 1:0, 2:0, 3:0, 4:0, 5:4, 6:15, 7:37, 8:87, 9:200}):
%o if n in adict:
%o return adict[n]
%o adict[n]=3*a(n-1)-2*a(n-2)+2*a(n-3)-4*a(n-4)+2*a(n-5)-2*a(n-6)-4*a(n-7)-a(n-8)+a(n-9)+2*a(n-10)
%o return adict[n] # _David Nacin_, Mar 07 2012
%Y Cf. A000045, A006498, A006500.
%K nonn,easy
%O 1,2
%A _Jeff Burch_
%E a(19) changed from 10416 to 10816 by _David Nacin_, Mar 04 2012