5 INTERACTION
Π engages in bilateral bargaining with its opponent
Ω. Π and Ω each exchange offers alternately at suc-
cessive discrete times (Kraus, 2001). They enter into
a commitment if one of them accepts a standing offer.
The protocol has three stages:
1. Simultaneous, initial, binding offers from both
agents;
2. A sequence of alternating offers, and
3. An agent quits and walks away from the negotia-
tion.
In the first stage, the agents simultaneously send Of-
fer(.) messages to each other that stand for the entire
negotiation. These initial offers are taken as limits
on the range of values that are considered possible.
This is crucial to the method described in Sec. 3.3
where there are domains that would otherwise be un-
bounded. The exchange of initial offers “stakes out
the turf” on which the subsequent negotiation will
take place. In the second stage, an Offer(.) message is
interpreted as an implicit rejection, Reject(.), of the
opponent’s offer on the table. Second stage offers
stand only if accepted by return — Π interprets these
offers as indications of Ω’s willingness to accept —
they are represented as beliefs with sentence proba-
bilities that decay in time. The negotiation ceases ei-
ther in the second round if one of the agents accepts a
standing offer or in the final round if one agent quits
and the negotiation breaks down.
To support the offer-exchange process, Π has do
two different things. First, it must respond to offers
received from Ω — that is described in Sec. 4. Sec-
ond, it must send offers, and possibly information,
to Ω. This section describes machinery for estimat-
ing the probabilities P(ΩAcc(δ)) where the predicate
ΩAcc(δ) means “Ω will accept Π’s offer δ”. In the
following, Π is attempting to purchase of a particular
second-hand motor vehicle, with some period of war-
ranty, for cash from Ω as described in Sec. 4.1. So a
deal δ will be represented by the pair (w, p) where w
is the period of warranty in years and $p is the price.
Π assumes the following two preference relations
for Ω, and K contains:
κ
11
:
∀x, y, z((x < y) → (ΩAcc(y, z) → ΩAcc(x, z)))
κ
12
:
∀x, y, z((x < y) → (ΩAcc(z, x) → ΩAcc(z, y)))
As in Sec. 4, these sentences conveniently reduce the
number of possible worlds. The two preference rela-
tions κ
11
and κ
12
induce a partial ordering on the sen-
tence probabilities in the P(ΩAcc(w, p)) array from
the top-left where the probabilities are ≈ 1, to the
bottom-right where the probabilities are ≈ 0. There
are fifty-one possible worlds that are consistent with
K.
Suppose that the offer exchange has proceeded as
follows: Ω asked for $6,900 with one year war-
ranty and Π refused, then Π offered $5,000 with
two years warranty and Ω refused, and then Ω asked
for $6,500 with three years warranty and Π re-
fused. Then at the next time step B contains: β
11
:
ΩAcc(3, [6.8, 7.0)), β
12
: ΩAcc(2, [5.0, 5.2)) and
β
13
: ΩAcc(1, [6.4, 6.6)), and with a 10% decay in
integrity for each time step: P(β
11
) = 0.7, P(β
12
) =
0.2 and P(β
13
) = 0.9
Eqn. 3 is used to calculate the distribution W
{K,B}
which shows that there are just five different probabil-
ities in it. The probability matrix for the proposition
ΩAcc(w, p) is:
p w 0 1 2 3 4
[7.0, ∞) 0.9967 0.9607 0.8428 0.7066 0.3533
[6.8, 7.0) 0.9803 0.9476 0.8330 0.7000 0.3500
[6.6, 6.8) 0.9533 0.9238 0.8125 0.6828 0.3414
[6.4, 6.6) 0.9262 0.9000 0.7920 0.6655 0.3328
[6.2, 6.4) 0.8249 0.8019 0.7074 0.5945 0.2972
[6.0, 6.2) 0.7235 0.7039 0.6228 0.5234 0.2617
[5.8, 6.0) 0.6222 0.6058 0.5383 0.4523 0.2262
[5.6, 5.8) 0.5208 0.5077 0.4537 0.3813 0.1906
[5.4, 5.6) 0.4195 0.4096 0.3691 0.3102 0.1551
[5.2, 5.4) 0.3181 0.3116 0.2846 0.2391 0.1196
[5.0, 5.2) 0.2168 0.2135 0.2000 0.1681 0.0840
In this array, the derived sentence probabilities for the
three sentences in B are shown in bold type; they are
exactly their given values.
Π’s negotiation strategy is a function S : K × B →
A where A is the set of actions that send Offer(.),
Accept(.), Reject(.) and Quit(.) messages to Ω. If Π
sends Offer(.), Accept(.) or Reject(.) messages to Ω
then she is giving Ω information about herself. In an
infinite-horizon bargaining game where there is no in-
centive to trade now rather than later, a self-interested
agent will “sit and wait”, and do nothing except, per-
haps, to ask for information. The well known bar-
gaining response to an approach by an interested party
“Well make me an offer” illustrates how a shrewd bar-
gainer may behave in this situation.
An agent may be motivated to act for various rea-
sons — three are mentioned. First, if there are
costs involved in the bargaining process due either
to changes in the value of the negotiation object with
time or to the intrinsic cost of conducting the nego-
tiation itself. Second, if there is a risk of breakdown
caused by the opponent walking away from the bar-
gaining table. Third, if the agent is concerned with
establishing a sense of trust (Ramchurn et al., 2003)
with the opponent —this could be the case in the es-
tablishment of a business relationship. Of these three
reasons the last two are addressed here. The risk of
breakdown may be reduced, and a sense of trust may
be established, if the agent appears to its opponent
to be “approaching the negotiation in an even-handed
manner”. One dimension of “appearing to be even-
handed” is to be equitable with the value of informa-
AN AGENT FOR EMERGENT PROCESS MANAGEMENT
9