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A270837
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Numbers n such that sigma(n-1)+phi(n-1) = (5n-7)/2.
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3
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3, 5, 7, 9, 17, 33, 65, 67, 129, 257, 513, 1025, 2049, 4097, 8193, 16385, 32769, 65537, 131073, 262145, 524289, 1048577, 2097153, 4194305, 8388609, 16777217, 33554433, 67108865, 134217729, 268435457, 536870913, 1073741825, 2147483649, 4294967297, 5606129563
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OFFSET
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1,1
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COMMENTS
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Numbers n such that A065387(n-1) = (5n-7)/2.
Numbers of the form 2^n + 1 for n >= 1 from A000051 are terms.
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LINKS
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EXAMPLE
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17 is a term because sigma(16)+phi(16) = 31+8 = 39 = (5*17-7)/2.
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MATHEMATICA
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Select[Range[10^6], DivisorSigma[1, # - 1] + EulerPhi[# - 1] == (5 # - 7)/2 &] (* Michael De Vlieger, Mar 24 2016 *)
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PROG
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(Magma) [n: n in[2..10^7] | 2*(SumOfDivisors(n-1) + EulerPhi(n-1)) eq 5*n-7]
(PARI) lista(nn) = {for(n=2, nn, if(sigma(n-1) + eulerphi(n-1) == (5*n-7)/2, print1(n, ", "))); } \\ Altug Alkan, Mar 23 2016
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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