A069970 - OEIS

%I #14 May 29 2024 14:43:38

%S 3,4,352,525,532,564,572,782,3783,5242,5762,5784,7852,7884,31732,

%T 38817,41736,46194,52942,57842,61146,63075,67266,68853,95418,196313,

%U 403194,424292,436642,444382,493592,521812,521853,521856,523682,527067,527452,541132,543442

%N Numbers k such that sigma(reverse(k)) = sigma(reverse(k-1)) + sigma(reverse(k-2)).

%H Michael S. Branicky, <a href="/A069970/b069970.txt">Table of n, a(n) for n = 1..138</a> (all terms < 10^8)

%e sigma(reverse(352)) = sigma(253) = 288 = 234 + 54 = sigma(153) + sigma(53) = sigma(reverse(352-1)) + sigma(reverse(352-2)).

%t rev[n_] := FromDigits[Reverse[IntegerDigits[n]]]; Select[Range[3, 10^5], DivisorSigma[1, rev[ # ]] == DivisorSigma[1, rev[ # - 1]] + DivisorSigma[1, rev[ # - 2]] &]

%o (Python)

%o from itertools import count, islice

%o from sympy import divisor_sigma as sigma

%o def agen(): # generator of terms

%o     dr2, dr1 = 1, 3

%o     for k in count(3):

%o         dr = sigma(int(str(k)[::-1]))

%o         if dr == dr1 + dr2:

%o             yield k

%o         dr2, dr1 = dr1, dr

%o print(list(islice(agen(), 25))) # _Michael S. Branicky_, May 29 2024

%Y Cf. A000203, A004086.

%K base,nonn

%O 1,1

%A _Joseph L. Pe_, Apr 29 2002

%E More terms from _Sean A. Irvine_, May 28 2024