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 A056481 #25 Feb 19 2024 01:48:47 %S A056481 0,0,0,1,1,3,2,7,6,14,12,31,27,63,56,123,120,255,238,511,495,1015,992, %T A056481 2047,2010,4092,4032,8176,8127,16383,16242,32767,32640,65503,65280, %U A056481 131061,130788,262143,261632,524223,523770,1048575,1047494,2097151,2096127,4194162 %N A056481 Number of primitive (aperiodic) palindromic structures using exactly two different symbols. %C A056481 Permuting the symbols will not change the structure. Identical to A056476 for n>1. %D A056481 M. R. Nester (1999). Mathematical investigations of some plant interaction designs. PhD Thesis. University of Queensland, Brisbane, Australia. [See A056391 for pdf file of Chap. 2] %F A056481 a(n) = A056476(n) - A000007(n) - A000007(n-1). %o A056481 (Python) %o A056481 from sympy import mobius, divisors %o A056481 def A056481(n): return sum(mobius(n//d)<<(d-1>>1) for d in divisors(n, generator=True)) if n>1 else 0 # _Chai Wah Wu_, Feb 18 2024 %Y A056481 Column 2 of A284826. %Y A056481 Cf. A056463. %K A056481 nonn %O A056481 0,6 %A A056481 _Marks R. Nester_ %E A056481 More terms (using A056476) from _Joerg Arndt_, May 22 2021