cp's OEIS Frontend

A368855 Index of the first occurrence of n in A076982.

%I A368855 #37 Jan 14 2024 20:36:53
%S A368855 1,2,3,8,14,15,39,20,44,35,195,119,104,594,224,384,455,539,440,560,
%T A368855 3080,2184,1539,2015,2639,5264,4199,15399,13299,8855,23919,2079,30744,
%U A368855 43680,36575,14399,5984,58695,113399,47124,107184,12375,78624,98175,73359,111320,242879,185724
%N A368855 Index of the first occurrence of n in A076982.
%H A368855 Robert G. Wilson v, <a href="/A368855/b368855.txt">Table of n, a(n) for n = 1..155</a>
%F A368855 a(n) = (sqrt(8*A076983(n)+1)-1)/2. - _Amiram Eldar_, Jan 10 2024
%e A368855 a(1) = 1 since A076982(1) = 1;
%e A368855 a(2) = 2 since A076982(2) = 2;
%e A368855 a(3) = 3 since A076982(3) = 3;
%e A368855 a(4) = 8 since A076982(8) = 4;
%e A368855 a(5) = 14 since A076982(14) = 5; etc.
%t A368855 k = 1; t[_] := 0; f[n_] := Length@ Select[ Divisors[n (n +1)/2], IntegerQ@ Sqrt[8# +1] &]; While[k < 250000, a = f@k; If[ t[a] == 0, t[a] = k]; k++]; t /@ Range@ 48
%Y A368855 Cf. A000217, A002378, A076982, A076983.
%Y A368855 Essentially the same as A225400.
%K A368855 nonn
%O A368855 1,2
%A A368855 _Robert G. Wilson v_, Jan 09 2024