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 A309901 #9 Aug 26 2019 11:24:09
%S A309901 0,1,7,25,52,52,538,1267,1267,1267,20950,20950,198097,1260979,1260979,
%T A309901 6043948,6043948,92137390,92137390,866978368,2029239835,5516024236,
%U A309901 26436730642,57817790251,246104147905,810963220867,1658251830310,6741983486968,21993178456942
%N A309901 Approximation of the 3-adic integer exp(-3) up to 3^n.
%C A309901 In p-adic field, the exponential function exp(x) is defined as Sum_{k>=0} x^k/k!. When extended to a function over the metric completion of the p-adic field, exp(x) has radius of convergence p^(-1/(p-1)) (i.e., exp(x) converges for x such that |x|_p < p^(-1/(p-1)), where |x|_p is the p-adic metric). As a result, for odd primes p, exp(p) is well-defined in p-adic field, and exp(4) is well defined in 2-adic field.
%C A309901 a(n) is the multiplicative inverse of A309900(n) modulo 3^n.
%H A309901 Wikipedia, <a href="https://en.wikipedia.org/wiki/P-adic_number">p-adic number</a>
%o A309901 (PARI) a(n) = lift(exp(-3 + O(3^n)))
%Y A309901 Cf. A309900.
%Y A309901 The 3-adic expansion of exp(-3) is given by A309866.
%Y A309901 Approximations of exp(-p) in p-adic field: this sequence (p=3), A309903 (p=5), A309905 (p=7).
%K A309901 nonn
%O A309901 0,3
%A A309901 _Jianing Song_, Aug 21 2019