[math-fun] 15 puzzle solved?

Patric.Ostergard@hut.fi writes as follows: a(4)=80. This has been shown in A. Br\"ungger, A. Marzetta, K. Fukuda & J. Nievergelt, The parallel search bench ZRAM and its applications, Annals of Operations Research 90 (1999), 45--63. NJAS