%I #34 Nov 22 2021 21:25:26

%S 1,3,29,901,89893,5758715,22687425

%N Order of all-2s configuration on the 2n X 2n sandpile grid graph.

%H Laura Florescu, Daniela Morar, David Perkinson, Nicholas Salter, Tianyuan Xu, Sandpiles and Dominos, <a href="/A256043/a256043.pdf">Illustration of recurrent identity element for the sandpile grid graph</a> [Figure 1 from Sandpiles and Dominos paper] [Le Borgne et al., 2002, show a very similar figure - see Fig. 7.]

%H Laura Florescu, Daniela Morar, David Perkinson, Nicholas Salter, Tianyuan Xu, <a href="http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i1p66">Sandpiles and Dominos</a>, Electronic Journal of Combinatorics, Volume 22(1), 2015. Also arXiv:1406.0100, May 31 2014. [Mentions this sequence together with a different sequence (A065072) with the same initial terms]

%H Le Borgne, Yvan, and Dominique Rossin, <a href="https://doi.org/10.1016/S0012-365X(02)00347-3">On the identity of the sandpile group</a>, Discrete mathematics 256.3 (2002): 775-790.

%H David Perkinson, <a href="http://people.reed.edu/~davidp/pcmi/lectures/15combined.pdf">Lecture 15: Sandpiles</a>, PCMI 2008 Undergraduate Summer School. Gives a(7).

%Y Cf. A065072, A256047.

%Y Bisection of main diagonal of A256045.

%K nonn,more

%O 1,2

%A _N. J. A. Sloane_, Mar 14 2015

%E a(7) added by _Scott R. Shannon_, Apr 23 2019