This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A118670 #4 Mar 30 2012 17:37:54 %S A118670 2,5,14,19,17,181,119,17,2459,121,89,181,545,59 %N A118670 Length of the shortest perfect modular pattern of type PMP(3,0) of n columns whose first row is 0...01 (with n-1 zeros). %C A118670 A modular pattern of type PMP(m,r) is a matrix of integers in the range 0 to (r-1) with the property that the sum of any element and its four adjacent elements is congruent to r (modulo m). The pattern is called perfect if no row or column is entirely zero. This generalizes the concept of perfect parity pattern introduced by D. E. Knuth in A118141. a(15) is greater than 4800. %e A118670 For 2 columns (n=2), if we start with the first row 0 1, it is found that successive additional rows such that the currently last row satisfies the PMP(3,0) condition %e A118670 are uniquely determined. This leads, after several steps, to %e A118670 0 1 %e A118670 2 2 %e A118670 2 1 %e A118670 1 1 %e A118670 2 0 %e A118670 Since, for the first time, the last (5th) row also satisfies the condition, we have found that the shortest PMP(3,0) matrix of 2 columns has 5 rows and thus a(2)=5. %Y A118670 Cf. A118141. %K A118670 nonn %O A118670 1,1 %A A118670 _John W. Layman_, May 19 2006