  In the study of topological indices two negative correlations are well known:
that between the number of subtrees and the Wiener index (sum of distances),
and that between the Merrifield-Simmons index (number of independent vertex
subsets) and the Hosoya index (number of independent edge subsets). That is,
among a certain class of graphs, the extremal graphs that maximize one index
usually minimize the other, and vice versa. In this paper, we first study the
numbers of subtrees in unicyclic graphs and unicyclic graphs with a given
girth, further confirming its opposite behavior to the Wiener index by
comparing with known results. We then consider the unicyclic graphs with a
given segment sequence and characterize the extremal structure with the maximum
number of subtrees. Furthermore, we show that these graphs are not extremal
with respect to the Wiener index. We also identify the extremal structures that
maximize the number of independent vertex subsets among unicyclic graphs with a
given segment sequence, and show that they are not extremal with respect to the
number of independent edge subsets. These results may be the first examples
where the above negative correlation failed in the extremal structures between
these two pairs of indices.
