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A117288
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Numbers k for which the cototient k-phi(k) is an octagonal number.
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7
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1, 2, 3, 5, 7, 11, 12, 13, 14, 16, 17, 19, 23, 29, 31, 37, 41, 43, 45, 47, 53, 57, 59, 61, 67, 71, 73, 76, 79, 83, 85, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 144, 149, 151, 157, 160, 163, 167, 173, 176, 179, 181, 184, 188, 191, 193, 197, 199, 211, 223
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OFFSET
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1,2
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LINKS
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EXAMPLE
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45 is in the sequence because 45-phi(45) = 21, which is an octagonal number.
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MATHEMATICA
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octQ[n_] := n == 0 || IntegerQ[(Sqrt[48*n + 16] + 4)/12]; Select[Range[250], octQ[# - EulerPhi[#]] &] (* Amiram Eldar, Mar 23 2021 *)
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PROG
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(PARI) isok(n) = ispolygonal(n - eulerphi(n), 8); \\ Michel Marcus, Feb 26 2014
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006
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EXTENSIONS
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STATUS
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approved
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