OFFSET
1,4
COMMENTS
The matrix M(n) is nonsingular (a(n) = n) only for n = 1, 5, 6 and 7 with determinant equal respectively to 1, 6561, 59049 and -531441.
MATHEMATICA
M[i_, j_]:=Total[IntegerDigits[i*j]]; Table[MatrixRank[Table[M[i, j], {i, n}, {j, n}]], {n, 69}]
PROG
(PARI) a(n)=matrank(matrix(n, n, i, j, sumdigits(i*j))) \\ Andrew Howroyd, Apr 08 2023
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Stefano Spezia, Apr 08 2023
STATUS
approved