Solution to Problem 226


A correct solution came from Will Cragoe and Bradley University alumnus Nathan Pauli.  Other correct solutions were submitted by Nancy Scharzkopf, Lou Cairoli, A. Teitelman.


 j  fj  Bound
n 2005  
n - 1 x  x > 0
n - 2 2005 - x  x < 2005
n - 3 2x - 2005  x > 1002
n - 4 4010 - 3x  x < 1337
n - 5 5x - 6015  x > 1203
n - 6 10025 - 8x  x < 1254
n - 7 13x - 16040  x > 1233
n - 8 26065 - 21x  x < 1242
n - 9 34x - 42105  x > 1238
n - 10 68170 - 55x  x < 1240
n - 11 89x - 110275  x > 1239

If the sequence is allowed to have negative terms, there is no longest sequence, so let's consider Fibonacci-like sequences having only positive terms. The longest Fibonacci-like sequence then has ten positive terms.  

If the n'th term of the sequence,  fn , is 2005 and  fn-1  is x, then the values of the previous terms of the sequence are given in the the table on the left, together with the bound on x needed for fj to be a positive integer. The bound  on  fn-11 is inconsistent with the others, so the longest sequence has ten terms and x = 1239, namely, 

25, 21, 46, 67, 113, 180, 293, 473, 766, 1239, 2005.  

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©2005 Alberto L. Delgado