FNOVERHALFCARTESIANPRODUCT
=LAMBDA(f,
    LAMBDA(n,
        LET(
            ixs, SEQUENCE(n, n, 1, 1),

            REM, "1-based indices.",
            x, 1 + MOD(ixs - 1, n),
            y, 1 + QUOTIENT(ixs - 1, n),

            IF(x >= y,
                f(x)(y),
                ""
            )
        )
    )
)