A055574 n satisfying sigma(n+1) = sigma(n-1).

34, 55, 285, 367, 835, 849, 919, 1241, 1505, 2911, 2914, 3305, 4149, 4188, 6111, 6903, 7170, 7913, 9360, 10251, 10541, 12566, 15086, 17273, 17815, 19005, 19689, 21411, 21462, 24882, 25020, 26610, 28125, 30593, 30789, 31485, 38211, 38983

Offset: 1

Author

Joseph L. Pe, Feb 12 2002

Keywords

• nonn

Comments

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

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

Examples

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

Links

• Vincenzo Librandi and Giovanni Resta, Table of n, a(n) for n = 1..10000 (first 200 terms from Vincenzo Librandi)

Crossrefs

Cf. A007373.

Programs

Mathematica

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

PARI

is(n)=sigma(n+1)==sigma(n-1) \\ Charles R Greathouse IV, Mar 09 2014

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