%I #21 May 09 2017 21:01:12

%S 34,55,285,367,835,849,919,1241,1505,2911,2914,3305,4149,4188,6111,

%T 6903,7170,7913,9360,10251,10541,12566,15086,17273,17815,19005,19689,

%U 21411,21462,24882,25020,26610,28125,30593,30789,31485,38211,38983

%N n satisfying sigma(n+1) = sigma(n-1).

%C Essentially the same as A007373: a(n) = A007373(n) + 1.

%C Numbers n such that antisigma(n+1) - antisigma(n-1) = 2*n + 1, where antisigma(m) = A024816(m) = sum of nondivisors of m. - _Jaroslav Krizek_, Mar 17 2013

%H Vincenzo Librandi and Giovanni Resta, <a href="/A055574/b055574.txt">Table of n, a(n) for n = 1..10000</a> (first 200 terms from Vincenzo Librandi)

%e sigma(34-1) = 48 = sigma(34+1), so 34 is a term of the sequence.

%t Select[Range[10^5], DivisorSigma[1, # + 1] == DivisorSigma[1, # - 1] &]

%o (PARI) is(n)=sigma(n+1)==sigma(n-1) \\ _Charles R Greathouse IV_, Mar 09 2014

%o (PARI) x=y=1; forfactored(z=3,10^6, if(sigma(z)==sigma(x), print1(y[1]", ")); x=y; y=z) \\ _Charles R Greathouse IV_, May 09 2017

%Y Cf. A007373.

%K nonn

%O 1,1

%A _Joseph L. Pe_, Feb 12 2002