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A067849
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a(n) = max{k: f(n),...,f^k(n) are prime}, where f(m) = 2m+1 and f^k denotes composition of f with itself k times.
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0
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2, 4, 1, 0, 3, 1, 0, 1, 1, 0, 2, 0, 0, 2, 1, 0, 0, 1, 0, 3, 1, 0, 1, 0, 0, 2, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 0, 2, 0, 0, 6, 0, 0, 0, 1, 0, 1, 1, 0, 1, 1, 0, 2, 0, 0, 0, 0
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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EXAMPLE
| f(2) = 5, f(f(2)) = 11, f(f(f(2))) = 23, f(f(f(f(2)))) = 47, all prime, but f^5(2) = 95 is not prime, so a(2) = 4.
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MATHEMATICA
| f[n_] := Module[{a = 2n + 1, i = 0}, While[PrimeQ[a], i++; a = 2a + 1]; i]; Table[f[i], {i, 1, 60}]
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CROSSREFS
| Sequence in context: A010586 A070678 A124091 * A164268 A152433 A094344
Adjacent sequences: A067846 A067847 A067848 * A067850 A067851 A067852
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KEYWORD
| nonn
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AUTHOR
| Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Feb 14 2002
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