In a recently published article by G. Ambrus et al. a new \emph{upper bound} for the density of an unit avoiding, periodic set is given as $0.2470$, the first upper bound $< 1/4$. A construction of Croft 1967 gave a \emph{lower bound} $δ_C = 0.22936$ for the density. To this date, no better construction with a higher bound has been given. In the \emph{first versions} of this article I gave a construction of planar sets with a "higher" density than Croft's tortoises. No explicit value for this density was given, it was just shown that Croft's density is a local minima in the density of the constructed 1-parameter family of planar sets. But now I found a servere error. After the correction in this article none of the investigated sets of constant diameter resulted in a new lower bound. I did not withdraw the article, maybe something could be useful for somebody.
On lower bounds of the density of planar periodic sets without unit distances
Moscow Institute of Physics and Technology（莫斯科物理技术学院）；Caucasus Mathematical Center of Adyghe State University（阿迪格国立大学高加索数学中心）；Skolkovo Institute of Science and Technology（斯科尔科沃科技学院）
Expected perimeter and area of the convex hull of planar Brownian motion stopped upon exiting the unit disk
University of Zagreb Faculty of Science（萨格勒布大学理学院）；Department of Mathematics and Statistics, Saint Louis University（圣路易斯大学数学与统计系）；University of Zagreb Faculty of Electrical Engineering and Computing（萨格勒布大学电气工程和计算学院）
