Function checkAt

test/compute-engine/derivatives.test.ts:721–725

(mathJson: any, f: (x: number) => number, x0: number)

    });
  });

  describe('BesselK (modified second kind)', () => {
    it('d/dx K_n(x) = -(K_{n-1}(x) + K_{n+1}(x))/2', () => {
      const expr = engine.expr(['D', ['BesselK', 'n', 'x'], 'x']);
      const result = expr.evaluate();
      expect(result.toString()).toMatchInlineSnapshot(
        `-1/2 * BesselK(n - 1, x) - 1/2 * BesselK(n + 1, x)`
      );