A049998 a(n) = b(n)-b(n-1), where b=A049997 are numbers of the form Fibonacci(i)*Fibonacci(j).
1, 1, 1, 1, 1, 1, 2, 1, 1, 3, 2, 1, 5, 3, 1, 1, 8, 5, 1, 2, 13, 8, 1, 1, 3, 21, 13, 2, 1, 5, 34, 21, 3, 1, 1, 8, 55, 34, 5, 1, 2, 13, 89, 55, 8, 1, 1, 3, 21, 144, 89, 13, 2, 1, 5, 34, 233, 144, 21, 3, 1, 1, 8, 55, 377, 233, 34, 5, 1, 2, 13, 89, 610, 377, 55, 8, 1, 1, 3, 21, 144, 987, 610, 89
Offset: 1
Keywords
A049999 a(n) = smallest index k such that Fibonacci(k) = d(n), where d = A049998 (sequence of first differences of ordered products of Fibonacci numbers, i.e., of A049997, with no duplicates).
1, 1, 1, 1, 1, 1, 3, 1, 1, 4, 3, 1, 5, 4, 1, 1, 6, 5, 1, 3, 7, 6, 1, 1, 4, 8, 7, 3, 1, 5, 9, 8, 4, 1, 1, 6, 10, 9, 5, 1, 3, 7, 11, 10, 6, 1, 1, 4, 8, 12, 11, 7, 3, 1, 5, 9, 13, 12, 8, 4, 1, 1, 6, 10, 14, 13, 9, 5, 1, 3, 7, 11, 15, 14, 10, 6, 1, 1, 4, 8, 12, 16, 15, 11
Offset: 1
Keywords
Comments
"David W. Wilson conjectured (Dec 14 2005) that" sequence A049998 "consists only of Fibonacci numbers. Proofs were found by Franklin T. Adams-Watters and Don Reble, Dec 14 2005." - Petros Hadjicostas, Nov 08 2019 [This comment was copied from A049998, which includes Don Reble's proof of the conjecture.]
Examples
From _Petros Hadjicostas_, Nov 08 2019: (Start) A049998(1) = 1 = Fibonacci(1) = Fibonacci(2), so a(1) = min(1,2) = 1. A049998(7) = 2 = Fibonacci(3), so a(7) = 3. A049998(10) = 3 = Fibonacci(4), so a(10) = 4. A049998(13) = 5 = Fibonacci(5), so a(13) = 5. A049998(17) = 8 = Fibonacci(6), so a(17) = 6. (End)
Links
- Clark Kimberling, Orderings of products of Fibonacci numbers, Fibonacci Quarterly 42:1 (2004), pp. 28-35. (Includes a proof of the conjecture proved in the Comments for sequence A049998.)
Formula
A000045(a(n)) = A049998(n) = A049997(n) - A049997(n-1) for n >= 1. - Petros Hadjicostas, Nov 08 2019
Extensions
Name edited by and more terms from Petros Hadjicostas, Nov 08 2019
A080097 a(n) = Fibonacci(n+2)^2 - 1.
0, 3, 8, 24, 63, 168, 440, 1155, 3024, 7920, 20735, 54288, 142128, 372099, 974168, 2550408, 6677055, 17480760, 45765224, 119814915, 313679520, 821223648, 2149991423, 5628750624, 14736260448, 38580030723, 101003831720
Offset: 0
Comments
a(n), a(n)+1 and a(n)+2 are consecutive members of A049997.
Links
- G. C. Greubel, Table of n, a(n) for n = 0..1000
- Sergio Falcon, On the Sequences of Products of Two k-Fibonacci Numbers, American Review of Mathematics and Statistics, March 2014, Vol. 2, No. 1, pp. 111-120.
- Index entries for linear recurrences with constant coefficients, signature (3,0,-3,1).
Crossrefs
Programs
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GAP
List([0..30], n-> Fibonacci(n+2)^2 -1); # G. C. Greubel, Jul 23 2019
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Magma
[Fibonacci(n+2)^2 -1: n in [0..30]]; // G. C. Greubel, Jul 23 2019
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Mathematica
CoefficientList[Series[(3x+2x^2-x^3)/((1-x^2)(1-2x-2x^2+x^3)), {x, 0, 30}], x] Table[Fibonacci[n+2]^2-1,{n,0,30}] (* Vladimir Joseph Stephan Orlovsky, Apr 03 2011 *) LinearRecurrence[{3,0,-3,1},{0,3,8,24},40] (* Harvey P. Dale, Nov 23 2024 *)
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Maxima
A080097(n):=fib(n+2)^2-1$ makelist(A080097(n),n,0,30); /* Martin Ettl, Nov 13 2012 */
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PARI
a(n)=fibonacci(n+2)^2-1 \\ Charles R Greathouse IV, Feb 06 2013
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Sage
[fibonacci(n+2)^2 -1 for n in (0..30)] # G. C. Greubel, Jul 23 2019
Formula
If n is odd, then a(n) = F(n+1)*F(n+3) = F(n)*F(n+4) - 2, else a(n) = F(n)*F(n+4) = F(n+1)*F(n+3) - 2, where F(n) = Fibonacci numbers (A000045).
a(n) = (Lucas(2*n+4) - 2*(-1)^n - 5)/5.
O.g.f.: x*(3-x)/((1-x^2)*(1-3*x+x^2)) (see a comment on A080144). - Wolfdieter Lang, Jul 30 2012
Sum_{n>=1} 1/a(n) = (43 - 15*sqrt(5))/18 = 29/9 - 5*phi/3, where phi is the golden ratio (A001622). - Amiram Eldar, Oct 20 2020
a(n) = 3*a(n-1) - 3*a(n-3) + a(n-4). - Joerg Arndt, Nov 13 2023
Extensions
Edited by Ralf Stephan, May 15 2005
A065108 Positive numbers expressible as a product of Fibonacci numbers.
1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 13, 15, 16, 18, 20, 21, 24, 25, 26, 27, 30, 32, 34, 36, 39, 40, 42, 45, 48, 50, 52, 54, 55, 60, 63, 64, 65, 68, 72, 75, 78, 80, 81, 84, 89, 90, 96, 100, 102, 104, 105, 108, 110, 117, 120, 125, 126, 128, 130, 135, 136, 144, 150, 156, 160, 162
Offset: 1
Keywords
Comments
There are infinitely many triples of consecutive terms of this sequence that are consecutive integers, see A065885. - John W. Layman, Nov 27 2001
Carmichael's theorem implies that 8 and 144 are the only Fibonacci numbers that are products of other Fibonacci numbers, cf. A235383. - Robert C. Lyons, Jan 13 2013
Examples
52 = 2 * 2 * 13 is the product of Fibonacci numbers 2, 2 and 13.
Links
- T. D. Noe, Table of n, a(n) for n = 1..10000
- Wawrzyniec Bieniawski, Piotr Masierak, Andrzej Tomski, and Szymon Ćukaszyk, Assembly Theory - Formalizing Assembly Spaces and Discovering Patterns and Bounds, Preprints.org (2025).
- Clemens Heuberger and Stephan Wagner, On the monoid generated by a Lucas sequence, arXiv:1606.02639 [math.NT], 2016.
- David A. Corneth, Table of 10000*i, a(10000*i), log(a(10000*i))/log(10000*i) for i = 1..470
Crossrefs
Programs
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Maple
with(combinat): A000045:=proc(n) options remember: RETURN(fibonacci(n)): end: mulfib:=proc(m,i) local j,q,f: f:=0: for j from i by -1 to 3 while(f=0) do if(irem(m, A000045(j))=0) then q:=iquo(m, A000045(j)): if(q=1) then RETURN(1) else f:=mulfib(q,j) fi fi od: RETURN(f): end: for i from 3 to 12 do for n from A000045(i) to A000045(i+1)-1 do m:=mulfib(n,i): if m=1 then printf("%d, ",n) fi od od: # C. Ronaldo
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Mathematica
nn = 1000; k = 1; fib = {}; While[k++; f = Fibonacci[k]; f <= nn, AppendTo[fib, f]]; s = fib; While[s2 = Select[Union[s, Flatten[Outer[Times, fib, s]]], # <= nn &]; Length[s2] > Length[s], s = s2]; s (* T. D. Noe, Jul 17 2012 *)
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PARI
list(lim)=if(lim<7, return([1..lim\1])); my(v=List([1]), F=List([2,3]), curfib, t, idx, newidx); while((t=F[#F]+F[#F-1])<=lim, listput(F,t)); F=setminus(Set(F), [8,144]); for(i=1,#F, curfib=F[i]; idx=1; while(v[idx]*curfib<=lim, newidx=#v+1; for(j=idx,#v, t=curfib*v[j]; if(t<=lim, listput(v,t))); idx=newidx)); Set(v) \\ Charles R Greathouse IV, Jun 15 2017
Formula
As Charles R Greathouse IV recently remarked, it would be good to have an asymptotic formula for this sequence. - N. J. A. Sloane, Jul 22 2012
Extensions
More terms from John W. Layman, Nov 27 2001
More terms from C. Ronaldo (aga_new_ac(AT)hotmail.com), Jan 02 2005
A271354 Products of two distinct Fibonacci numbers, both greater than 1.
6, 10, 15, 16, 24, 26, 39, 40, 42, 63, 65, 68, 102, 104, 105, 110, 165, 168, 170, 178, 267, 272, 273, 275, 288, 432, 440, 442, 445, 466, 699, 712, 714, 715, 720, 754, 1131, 1152, 1155, 1157, 1165, 1220, 1830, 1864, 1869, 1870, 1872, 1885, 1974, 2961, 3016
Offset: 1
Comments
For n > 5, the numbers F(i)*F(j) satisfying F(n-1) <= F(i)*F(j) <= F(n) also satisfy F(n-1) < F(i)*F(j) < F(n). They are the numbers for which i + j = n + 1, where 2 < i < j, so that the number of such F(i)*F(j) is floor(n/2) - 2. The least is 3*F(n-3) and the greatest is 2*F(n-2).
Examples
2*3 = 6, 2*5 = 10, 3*5 = 15, 2*8 = 16.
Links
- Clark Kimberling, Table of n, a(n) for n = 1..1000
- Clark Kimberling, Orderings of products of Fibonacci numbers, Fibonacci Quarterly 42:1 (2004), pp. 28-35.
Programs
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Mathematica
z = 200; f[n_] := Fibonacci[n]; Take[Sort[Flatten[Table[f[m] f[n], {n, 3, z}, {m, 3, n - 1}]]], 100] Times@@@Subsets[Fibonacci[Range[3,20]],{2}]//Union (* Harvey P. Dale, Jul 12 2025 *)
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PARI
list(lim)=my(v=List,F=vector(A130233(lim\2),k,fibonacci(k)),t); for(i=2,#F, for(j=1,i-1, t=F[i]*F[j]; if(t>lim,break); listput(v,t))); Set(v) \\ Charles R Greathouse IV, Oct 07 2016
A059389 Sums of two nonzero Fibonacci numbers.
2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 16, 18, 21, 22, 23, 24, 26, 29, 34, 35, 36, 37, 39, 42, 47, 55, 56, 57, 58, 60, 63, 68, 76, 89, 90, 91, 92, 94, 97, 102, 110, 123, 144, 145, 146, 147, 149, 152, 157, 165, 178, 199, 233, 234, 235, 236, 238, 241, 246, 254, 267
Offset: 1
Comments
The sums of two distinct nonzero Fibonacci numbers is essentially the same sequence: 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 16, 18, 21, ... (only 2 is missing), since F(i) + F(i) = F(i-2) + F(i+1). - Colm Mulcahy, Mar 02 2008
To elaborate on Mulcahy's comment above: all terms of A078642 are in this sequence; those are numbers with two distinct representations as the sum of two Fibonacci numbers, which are, as Alekseyev proved, numbers of the form 2*F(i) greater than 2. - Alonso del Arte, Jul 07 2013
Examples
10 is in the sequence because 10 = 2 + 8. 11 is in the sequence because 11 = 3 + 8. 12 is not in the sequence because no pair of Fibonacci numbers adds up to 12.
Links
- T. D. Noe, Table of n, a(n) for n = 1..1000
Crossrefs
Programs
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Maple
N:= 1000: # to get all terms <= N R:= NULL: for j from 1 do r:= combinat:-fibonacci(j); if r > N then break fi; R:= R, r; end: R:= {R}: select(`<=`, {seq(seq(r+s, s=R),r=R)},N); # if using Maple 11 or earlier, uncomment the next line # sort(convert(%,list)); # Robert Israel, Feb 15 2015
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Mathematica
max = 13; Select[Union[Total/@Tuples[Fibonacci[Range[2, max]], {2}]], # <= Fibonacci[max] &] (* Harvey P. Dale, Mar 13 2011 *)
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PARI
list(lim)=my(upper=log(lim*sqrt(5))\log((1+sqrt(5))/2)+1, t, tt, v=List([2])); if(fibonacci(t)>lim,t--); for(i=3,upper, t=fibonacci(i); for(j=2,i-1,tt=t+fibonacci(j); if(tt>lim, break, listput(v,tt)))); vecsort(Vec(v)) \\ Charles R Greathouse IV, Jul 24 2012
Formula
a(1) = 2 and for n >= 2 a(n) = F_(trinv(n-2)+2) + F_(n-((trinv(n-2)*(trinv(n-2)-1))/2)) where F_n is the n-th Fibonacci number, F_1 = 1 F_2 = 1 F_3 = 2 ... and the definition of trinv(n) is in A002262. - Noam Katz (noamkj(AT)hotmail.com), Feb 04 2001
log a(n) ~ sqrt(n log phi) where phi is the golden ratio A001622. There are (log x/log phi)^2 + O(log x) members of this sequence up to x. - Charles R Greathouse IV, Jul 24 2012
Extensions
More terms from Larry Reeves (larryr(AT)acm.org), Jan 31 2001
A272909 Numbers that are the product of two Lucas numbers L(i), for i >= 1, using the Lucas numbers as defined in A000204.
1, 3, 4, 7, 9, 11, 12, 16, 18, 21, 28, 29, 33, 44, 47, 49, 54, 72, 76, 77, 87, 116, 121, 123, 126, 141, 188, 198, 199, 203, 228, 304, 319, 322, 324, 329, 369, 492, 517, 521, 522, 532, 597, 796, 836, 841, 843, 846, 861, 966, 1288, 1353, 1363, 1364, 1368, 1393
Offset: 1
Comments
Conjecture: if c and d are consecutive terms, then d - c is a product of two Lucas numbers or a product of two Fibonacci numbers.
Links
- Clark Kimberling, Table of n, a(n) for n = 1..1000
Programs
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Mathematica
Take[Union@Flatten@Table[LucasL[i] LucasL[j], {i, 0, 15}, {j, i}], 60] (* adapted by Vincenzo Librandi, Sep 04 2016 *)
A094563 Triple products of Fibonacci numbers: F(i)*F(j)*F(k), 2 <= i <= j <= k.
1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 13, 15, 16, 18, 20, 21, 24, 25, 26, 27, 30, 32, 34, 39, 40, 42, 45, 48, 50, 52, 55, 63, 64, 65, 68, 72, 75, 78, 80, 84, 89, 102, 104, 105, 110, 117, 120, 125, 126, 128, 130, 136, 144, 165, 168, 169, 170, 178, 189, 192
Offset: 1
Keywords
Comments
This sequence contains A049997 as a subsequence (aside from its first term), so a(n) << sqrt(phi)^n. All Fibonacci factors must be at most the number, so a(n) >> (phi^(1/3))^n. - Charles R Greathouse IV, Feb 06 2013
Examples
F(2)*F(2)*F(2) = 1 < F(2)*F(2)*F(3) = 2 < ... < F(4)*F(4)*F(4) = 27 < F(3)*F(4)*F(5) = 30 < F(3)*F(3)*F(6) = 32 < ...
Links
- Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
- C. Kimberling, Orderings of products of Fibonacci numbers, Fibonacci Quart. 42:1 (2004), pp. 28-35.
Programs
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Mathematica
Select[Union[Times@@@Tuples[Fibonacci[Range[12]],3]],#<200&] (* Harvey P. Dale, Dec 13 2011 *)
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PARI
list(lim)=my(phi=(1+sqrt(5))/2, v=vector(log(lim*sqrt(5))\log(phi),i,fibonacci(i+1)), u=List(), t, t1); for(i=1,#v, for(j=i,#v, t1=v[i]*v[j];if(t1>lim,break); for(k=j, #v, t=t1*v[k]; if(t>lim,break,listput(u,t))))); vecsort(Vec(u),,8) \\ Charles R Greathouse IV, Feb 06 2013
A226857 Numbers that are both the sum of two Fibonacci numbers and the product of two Fibonacci numbers.
0, 1, 2, 3, 4, 5, 6, 8, 9, 10, 13, 15, 16, 21, 24, 26, 34, 39, 42, 55, 63, 68, 89, 102, 110, 144, 165, 178, 233, 267, 288, 377, 432, 466, 610, 699, 754, 987, 1131, 1220, 1597, 1830, 1974, 2584, 2961, 3194, 4181, 4791, 5168, 6765, 7752, 8362, 10946, 12543, 13530
Offset: 1
Comments
All Fibonacci numbers are in the sequence. The only prime numbers in this sequence are prime Fibonacci numbers.
Examples
5 + 21 = 2 * 13 = 26, therefore 26 is in the sequence. 8 + 21 = 1 * 34 = 34, therefore 34 is in the sequence. 5 + 34 = 3 * 13 = 39, therefore 39 is in the sequence.
Links
- T. D. Noe, Table of n, a(n) for n = 1..1000
Programs
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Mathematica
t = Fibonacci[Range[0, 25]]; t1 = Select[Union[Flatten[Table[a + b, {a, t}, {b, t}]]], # <= t[[-1]] &]; t2 = Select[Union[Flatten[Table[a*b, {a, t}, {b, t}]]], # <= t[[-1]] &]; Intersection[t1, t2] (* T. D. Noe, Jul 03 2013 *)
Formula
Conjecture: a(n) = a(n-3)+a(n-6) for n>12. - Colin Barker, Nov 09 2014
Empirical g.f.: -x^2*(x^10 +x^9 +x^8 +2*x^7 +3*x^6 +3*x^5 +3*x^4 +3*x^3 +3*x^2 +2*x +1) / (x^6 +x^3 -1). - Colin Barker, Nov 09 2014
A272900 Fibonacci-products fractal sequence.
1, 1, 1, 2, 1, 2, 1, 3, 2, 1, 3, 2, 1, 3, 4, 2, 1, 3, 4, 2, 1, 3, 5, 4, 2, 1, 3, 5, 4, 2, 1, 3, 5, 6, 4, 2, 1, 3, 5, 6, 4, 2, 1, 3, 5, 7, 6, 4, 2, 1, 3, 5, 7, 6, 4, 2, 1, 3, 5, 7, 8, 6, 4, 2, 1, 3, 5, 7, 8, 6, 4, 2, 1, 3, 5, 7, 9, 8, 6, 4, 2, 1, 3, 5, 7, 9
Offset: 1
Comments
Let F = A000045, the Fibonacci numbers. Let s be the sequence of all products F(i)F(j), for 2 <= i < = j, arranged in increasing order; viz., (1,2,3,4,5,6,8,9,10,13,15,...) = (F(2)F(2), F(2)F(3), F(2)F(4), F(3)F(3), F(2)F(5), ... ), as at A049997. The sequence of first factors is (F(2), F(2), F(2), F(3), F(2),...), represented by indices (2,2,2,3,2,...). Subtracting 1 from each term leaves A272900, which is a fractal sequence; i.e., the removal of the first occurrence of each term in A272900 leaves A272900, so that the sequence contains itself infinitely many times.
Links
- Clark Kimberling, Table of n, a(n) for n = 1..1000
- Clark Kimberling, Orderings of products of Fibonacci numbers, Fibonacci Quarterly 42:1 (2004), pp. 28-35.
Crossrefs
Programs
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Mathematica
z = 200; f[n_] := Fibonacci[n + 1]; u1 = Table[f[n], {n, 1, z}]; u2 = Sort[Flatten[Table[f[i]*f[j], {i, 1, z}, {j, i, z}]]]; Table[Select[Range[30], MemberQ[u1, u2[[i]]/f[#]] &][[1]], {i, 1, z}]
Comments
Links
Crossrefs
Programs
Mathematica
Extensions