In general, *(b - 1) / b* of the nodes of a search tree are on the bottom row.
If the branching factor is *b = 2*, half the nodes are on the
bottom; with a higher branching factor, the proportion on the
bottom row is higher.

Korf calculates the work done by iterative deepening as
*b ^{d} * (1 - 1/b)^{-2}*,
where the multiplier approaches 1 as

My calculation of the work multiplier for iterative deepening is
*(b + 1) / (b - 1)*, which is not far from Korf's result.
The multiplier is a constant, independent of depth.

b | multiplier |

2 | 3.00 |

3 | 2.00 |

4 | 1.67 |

5 | 1.50 |

10 | 1.22 |