摘要: In 1975, P. Erd {o}s proposed the problem of determining the
maximum number $f(n)$ of edges in a graph with $n$ vertices in which
any two cycles are of different
lengths. In this paper, it is proved that $$f(n) geq n+ frac{107}{3}t+ frac{7}{3}$$
for $t=1260r+169 , (r geq 1)$
and $n geq frac{2119}{4}t^{2}+87978t+ frac{15957}{4}$. Consequently,
$ liminf sb {n to infty} {f(n)-n over sqrt n} geq sqrt {2 +
frac{7654}{19071}},$ which is better than the previous bounds
$ sqrt 2$ Y. Shi, Discrete Math. 71(1988), 57-71 , $ sqrt {2.4}$
C. Lai, Australas. J. Combin. 27(2003), 101-105 .
The conjecture $ lim_{n rightarrow infty} {f(n)-n over sqrt n}= sqrt {2.4}$ is not true.