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 A289905 #9 Jun 07 2024 10:42:27 %S A289905 0,1,2,3,-1,6,9,5,7,18,27,-1,-1,-1,54,4,11,15,21,19,8,81,-1,33,-1,57, %T A289905 -1,162,243,29,-1,45,63,-1,55,486,729,-1,87,-1,-1,-1,165,-1,1458,10, %U A289905 83,249,99,22,17,171,489,163,20,2187,-1,-1,-1,135,-1,189,-1,-1 %N A289905 Square array T(n,k) (n>0, k>0) read by antidiagonals: if gcd(n,k)>1 then T(n,k)=-1, otherwise T(n,k) = the unique m such that A289815(m) = n and A289816(m) = k. %C A289905 This sequence, when restricted to the pairs of coprime numbers, is the inverse of the function n -> (A289815(n), A289816(n)). %C A289905 If n and k are coprime, then the number of nonzero digits of the ternary representation of T(n,k) equals the number of distinct prime factors of n*k. %H A289905 Rémy Sigrist, <a href="/A289905/b289905.txt">Table of n, a(n) for n = 1..5050</a> %H A289905 Rémy Sigrist, <a href="/A289905/a289905.gp.txt">PARI program for A289905</a> %e A289905 The table begins: %e A289905 x\y: 1 2 3 4 5 6 7 8 ... %e A289905 1: 0 2 6 18 54 8 162 486 ... %e A289905 2: 1 -1 7 -1 19 -1 55 -1 ... %e A289905 3: 3 5 -1 21 57 -1 165 489 ... %e A289905 4: 9 -1 15 -1 63 -1 171 -1 ... %e A289905 5: 27 11 33 45 -1 17 189 513 ... %e A289905 6: 4 -1 -1 -1 22 -1 58 -1 ... %e A289905 7: 81 29 87 99 135 35 -1 567 ... %e A289905 8: 243 -1 249 -1 297 -1 405 -1 ... %e A289905 ... %o A289905 (PARI) \\ See Links section. %Y A289905 Cf. A289815, A289816. %K A289905 sign,tabl,base %O A289905 1,3 %A A289905 _Rémy Sigrist_, Jul 14 2017