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 A308482 #9 Jul 18 2019 07:46:13
%S A308482 4,9,20,25,27,40,49,80,81,121,169,189,243,272,289,361,369,400,416,470,
%T A308482 529,544,567,729,841,961,1071,1323,1369,1539,1681,1849,2000,2187,2209,
%U A308482 2809,2889,3213,3481,3721,4489,4617,5041,5329,6241,6561,6889,7749,7921,8667
%N A308482 Composites c such that T_{c-1} == (c/3)*3^(c-1) (mod c), where T_i denotes the i-th central trinomial coefficient (A002426) and (/) denotes the Kronecker symbol.
%C A308482 Composites satisfying a weaker version of an analog to a congruence satisfied by all primes > 3 (cf. Cao, Sun, 2015, Theorem 1.1 (i); cf. Sun, 2011, Remark to Conjecture A69).
%C A308482 Up to 9000, 189 is the only composite satisfying the congruence modulo c^2. Do any other such composites exist?
%H A308482 Hui-Qin Cao and Zhi-Wei Sun, <a href="https://doi.org/10.4064/cm139-1-8">Some congruences involving binomial coefficients</a>, Colloquium Mathematicum 139 (2015), 127-136, arXiv:<a href="https://arxiv.org/abs/1006.3069">1006.3069</a> [math.NT], 2010-2015.
%H A308482 Zhi-Wei Sun, <a href="https://arxiv.org/abs/0911.5665">Open Conjectures on Congruences</a>, arXiv:0911.5665 [math.NT], 2011.
%t A308482 aQ[n_] := CompositeQ[n] && Divisible[3^(n-1)*(Hypergeometric2F1[1/2, 1-n, 1, 4/3] - JacobiSymbol[n,3]) ,n]; Select[Range[1000], aQ] (* _Amiram Eldar_, Jul 10 2019 *)
%o A308482 (PARI) t(n) = sum(k=0, floor(n/2), binomial(n, k)*binomial(n-k, k))
%o A308482 is(n) = Mod(t(n-1), n)==kronecker(n, 3)*3^(n-1)
%o A308482 forcomposite(c=1, , if(is(c), print1(c, ", ")))
%Y A308482 Cf. A002426.
%K A308482 nonn
%O A308482 1,1
%A A308482 _Felix Fröhlich_, May 30 2019
%E A308482 a(45)-a(50) from _Amiram Eldar_, Jul 10 2019