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 A309905 #12 Aug 26 2019 11:22:00
%S A309905 0,1,43,190,1562,6364,56785,645030,3115659,14645261,14645261,
%T A309905 297120510,8206427482,22047714683,118936725090,118936725090,
%U A309905 23856744274805,123555535983608,588816563958022,5474057357689369,51069638099181941,51069638099181941
%N A309905 Approximation of the 7-adic integer exp(-7) up to 7^n.
%C A309905 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 A309905 a(n) is the multiplicative inverse of A309904(n) modulo 7^n.
%H A309905 Wikipedia, <a href="https://en.wikipedia.org/wiki/P-adic_number">p-adic number</a>
%o A309905 (PARI) a(n) = lift(exp(-7 + O(7^n)))
%Y A309905 Cf. A309904.
%Y A309905 The 7-adic expansion of exp(-7) is given by A309988.
%Y A309905 Approximations of exp(-p) in p-adic field: A309901 (p=3), A309903 (p=5), this sequence (p=7).
%K A309905 nonn
%O A309905 0,3
%A A309905 _Jianing Song_, Aug 21 2019