A069143 - OEIS

%I #9 Mar 05 2025 21:21:04

%S 1,1,3,7,18,31,57,180,320,1092,2478,10368,25704,100800,393120,1965600,

%T 10326624,39657600,131580288,511223790,1469266560,4911616440,

%U 22120488000,95841111600,460020244320,2163425531904,9805180170240

%N a(1) = a(2) = 1; a(n) = sigma(a(n-1)+a(n-2)).

%H Robert Israel, <a href="/A069143/b069143.txt">Table of n, a(n) for n = 1..138</a>

%p f:= proc(n) option remember; numtheory:-sigma(procname(n-1)+procname(n-2)) end proc:

%p f(1):= 1: f(2):= 1:

%p seq(f(n),n=1..40); # _Robert Israel_, Mar 05 2025

%t a[1] = a[2] = 1; a[n_] := a[n] = DivisorSigma[1, a[n - 1] + a[n - 2]]; Table[a[n], {n, 27}]

%t RecurrenceTable[{a[1]==a[2]==1,a[n]==DivisorSigma[1,a[n-1]+a[n-2]]},a,{n,30}] (* _Harvey P. Dale_, Feb 15 2015 *)

%K nonn,changed

%O 1,3

%A _Joseph L. Pe_, Apr 08 2002

%E Edited and extended by _Robert G. Wilson v_, Apr 11 2002