@article{Babic1993, 
     author = "Babi\'{c}, Darko and Graovac, Ante", 
     doi = "10.1016/0166-218X(93)90055-S", 
     issn = "0166218X", 
     journal = "Discrete Applied Mathematics", 
     link = "http://www.sciencedirect.com/science/article/pii/0166218X9390055S", 
     month = "aug", 
     number = "2", 
     pages = "117--123", 
     title = "Enumeration of Acyclic Walks in a Graph", 
     uri = "http://www.sciencedirect.com/science/article/pii/0166218X9390055S", 
     volume = "45", 
     year = "1993", 
}

Problems: 45