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A270836
Numbers n such that sigma(n-1) - phi(n-1) = (3n-5)/2.
3
3, 5, 9, 11, 17, 33, 65, 129, 231, 257, 513, 1025, 2049, 4097, 8193, 16385, 32769, 65537, 119831, 131073, 262145, 524289, 1048577, 2097153, 4194305, 8388609, 16777217, 33554433, 67108865, 134217729, 268435457, 536870913, 1073741825, 2147483649, 4294967297
OFFSET
1,1
COMMENTS
Numbers n such that A051612(n-1) = (3n-5)/2.
Numbers of the form 2^n + 1 for n >= 1 from A000051 are terms.
Prime terms are in A270778.
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..46 (terms < 10^13)
EXAMPLE
17 is a term because sigma(16) - phi(16) = 31-8 = 23 = (3*17-5)/2.
MATHEMATICA
Select[Range[10^6], 2 (DivisorSigma[1, # - 1] - EulerPhi[# - 1]) == 3 # - 5 &] (* Michael De Vlieger, Mar 24 2016 *)
PROG
(Magma) [n: n in[1..10^7] | 2*(SumOfDivisors(n-1) - EulerPhi(n-1)) eq 3*n-5]
(PARI) lista(nn) = {for(n=2, nn, if(sigma(n-1) - eulerphi(n-1) == (3*n-5)/2, print1(n, ", "))); } \\ Altug Alkan, Mar 23 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Mar 23 2016
EXTENSIONS
a(30)-a(32) from Michel Marcus, Apr 05 2016
a(33)-a(35) from Giovanni Resta, Apr 11 2016
STATUS
approved