Apéry congruence conjecture for the second-kind numbers modulo 99

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For n≥0n\ge 0, define the Apéry numbers of the second kind by

An(3)=∑k=0n(nk)2(n+kk)2.A^{(3)}_n=\sum_{k=0}^n \binom{n}{k}^2\binom{n+k}{k}^2.

Write the ternary expansion of nn as a finite word over {0,1,2}\{0,1,2\}, and let kk be a nonnegative integer.

Second-kind Apéry congruence conjecture. The numbers An(3)A^{(3)}_n satisfy the following congruences modulo 99:

  1. An(3)≡1(mod9)A^{(3)}_n\equiv1\pmod 9 if and only if the ternary expansion of nn contains 6k6k digits equal to 11 for some kk, and otherwise contains only 00's and 22's.
  2. An(3)≡2(mod9)A^{(3)}_n\equiv2\pmod 9 if and only if the ternary expansion of nn contains 6k+56k+5 digits equal to 11 for some kk, and otherwise contains only 00's and 22's.
  3. An(3)≡4(mod9)A^{(3)}_n\equiv4\pmod 9 if and only if the ternary expansion of nn contains 6k+46k+4 digits equal to 11 for some kk, and otherwise contains only 00's and 22's.
  4. An(3)≡5(mod9)A^{(3)}_n\equiv5\pmod 9 if and only if the ternary expansion of nn contains 6k+16k+1 digits equal to 11 for some kk, and otherwise contains only 00's and 22's.
  5. An(3)≡7(mod9)A^{(3)}_n\equiv7\pmod 9 if and only if the ternary expansion of nn contains 6k+26k+2 digits equal to 11 for some kk, and otherwise contains only 00's and 22's.
  6. An(3)≡8(mod9)A^{(3)}_n\equiv8\pmod 9 if and only if the ternary expansion of nn contains 6k+36k+3 digits equal to 11 for some kk, and otherwise contains only 00's and 22's.
  7. In all other cases, An(3)A^{(3)}_n is divisible by 99; in particular, An(3)≢3,6(mod9)A^{(3)}_n\not\equiv3,6\pmod 9 for every nn.

As with the first-kind numbers, the relevant differential equation is not suitable for the paper's method. The patterns were obtained conjecturally modulo 99, and the supplied text gives no resolution.

References

Primary source

Christian Krattenthaler and Thomas W. Müller, “A method for deterining the mod-3^k behaviour of recursive sequences”, arXiv:1308.2856 (2013).

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