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A202211
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a(1)=2, a(2)=3, for n >= 3, a(n) = 2*(gpd(a(n-1)) + gpd(a(n-2))) + 1, where gpd(n) is the greatest prime divisor of n.
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2
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2, 3, 11, 29, 81, 65, 33, 49, 37, 89, 253, 225, 57, 49, 53, 121, 129, 109, 305, 341, 185, 137, 349, 973, 977, 2233, 2013, 181, 485, 557, 1309, 1149, 801, 945, 193, 401, 1189, 885, 201, 253, 181, 409, 1181, 3181, 8725, 7061, 1313, 817, 289
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OFFSET
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1,1
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COMMENTS
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The question about the boundedness of the sequence is equivalent to the question about its eventually periodicity. For example, the sequence defined by the same recurrence relation and initial values b(1)=2 and b(2)=5 is 2, 5, 15, 21, 25, 25, 21, 25, 25, ... so is periodic for n>=4 with the period {21,25,25}.
Problem. (a) Do there exist initial terms a(1) and a(2) depending on a given N for which the sequence has the least period of length >= N? (b) Do there exist initial terms a(1) and a(2) for which the sequence has no period?
Conjecture. Problem (a) is answered in affirmative, while problem (b) is answered in the negative.
This sequence is eventually periodic and therefore bounded: a(61)=a(85)=85 [sic] and a(62)=a(86)=73. [D. S. McNeil, Dec 14 2011]
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LINKS
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Table of n, a(n) for n=1..49.
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MATHEMATICA
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a[1] := 2; a[2] := 3; a[n_] := a[n] = 2(FactorInteger[a[n - 1]][[-1, 1]] + FactorInteger[a[n - 2]][[-1, 1]]) + 1; Table[a[n], {n, 50}] (* Alonso del Arte, Dec 14 2011 *)
nxt[{a_, b_}]:={b, 2(FactorInteger[a][[-1, 1]]+FactorInteger[b] [[-1, 1]])+ 1}; Transpose[NestList[nxt, {2, 3}, 120]][[1]] (* Harvey P. Dale, Dec 15 2011 *)
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CROSSREFS
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Cf. A006530
Sequence in context: A237038 A309701 A243896 * A104081 A267902 A003455
Adjacent sequences: A202208 A202209 A202210 * A202212 A202213 A202214
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KEYWORD
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nonn
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AUTHOR
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Vladimir Shevelev, Dec 14 2011
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STATUS
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approved
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