A140481 a(1) = 1; for n >= 1, a(n+1) is obtained by adding to a(n) the a(n)-th smallest number not dividing a(n).

1, 3, 8, 20, 46, 96, 204, 420, 864, 1752, 3520, 7068, 14160, 28360, 56736, 113508, 227040, 454176, 908424, 1816944, 3633908, 7267828, 14535662, 29071328, 58142704, 116285418, 232570884, 465141864, 930283760, 1860567600

Offset

1,2

Links

Table of n, a(n) for n=1..30.

Formula

a(n+1) = 2*a(n) + tau(a(n)) (cf. A000005). - Hans Havermann and Franklin T. Adams-Watters, Jun 25 2008. Using a(n+1) = a(n) + tau(a(n)) would give A064491.

Example

The smallest number not dividing 1 is 2, so a(2) = 1+2 = 3.
The numbers not dividing 3 are 2, 4, 5, 6, ..., so a(3) = 3+5 = 8.
The numbers not dividing 8 are 3, 5, 6, 7, 9, 10, 11, 12, ..., so a(4) = 8+12 = 20.

Crossrefs

Cf. A000005, A064491, A140482.

Sequence in context: A134393 A034504 A191522 * A027928 A026624 A026690

Adjacent sequences: A140478 A140479 A140480 * A140482 A140483 A140484

Keyword

nonn

Author

Eric Angelini, Jun 25 2008

Extensions

More terms from Hans Havermann, Jun 25 2008

Status

approved