Eric Weisstein's World of Mathematics, <a href="httphttps://mathworld.wolfram.com/PalindromicNumber.html">Palindromic Number</a>.
Eric Weisstein's World of Mathematics, <a href="httphttps://mathworld.wolfram.com/Ternary.html">Ternary</a>.
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Aayush Rajasekaran, Jeffrey Shallit, and Tim Smith, <a href="https://arxiv.org/abs/1706.10206">Sums of palindromes: an approach via automata</a>, arXiv:1706.10206 [cs.FL], 2017.
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return int((c:=n-x)*x+int(digits(c, 3)[-2::-1]or'0', 3) if n<x+y else (c:=n-y)*y+int(digits(c, 3)[-1::-1]or'0', 3)) # Chai Wah Wu, Jun 13 2024