A368855 Index of the first occurrence of n in A076982.

1, 2, 3, 8, 14, 15, 39, 20, 44, 35, 195, 119, 104, 594, 224, 384, 455, 539, 440, 560, 3080, 2184, 1539, 2015, 2639, 5264, 4199, 15399, 13299, 8855, 23919, 2079, 30744, 43680, 36575, 14399, 5984, 58695, 113399, 47124, 107184, 12375, 78624, 98175, 73359, 111320, 242879, 185724

Offset: 1

Author

Robert G. Wilson v, Jan 09 2024

Keywords

• nonn

Examples

			a(1) = 1 since A076982(1) = 1;
a(2) = 2 since A076982(2) = 2;
a(3) = 3 since A076982(3) = 3;
a(4) = 8 since A076982(8) = 4;
a(5) = 14 since A076982(14) = 5; etc.
		

Links

• Robert G. Wilson v, Table of n, a(n) for n = 1..155

Crossrefs

Cf. A000217, A002378, A076982, A076983.

Essentially the same as A225400.

Programs

Mathematica

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

Formula

a(n) = (sqrt(8*A076983(n)+1)-1)/2. - Amiram Eldar, Jan 10 2024