Clear a sum with the root of a quadratic below the bar by its conjugate

BREAKING-CHANGES.md

@@ -1058,6 +1058,25 @@ secant too now, and the integrand is `sgn(cos(x)) (1 + u^2)^(3/2)/(a + b u)^2` i
 | `"sec(x)^7/(a + b*tan(x))^3".ToEntity().Integrate("x")` | `integral(...)` | 1,253 characters |
 | `"sec(x)/(a + b*cot(x))".ToEntity().Integrate("x")` | `integral(...)` | 571 characters |
 
+### A sum with the root of a quadratic below the bar is cleared by its conjugate
+
+**Answers where there were none.** `1/(a c + b c x^2 + d sqrt(a + b x^2))` was declined: Euler's substitution
+writes `sqrt(b)` into every coefficient for a symbolic leading coefficient, and the rational integrator could not
+finish. `(A + B sqrt(Q))/(C + D sqrt(Q))` is `(A + B sqrt(Q)) (C - D sqrt(Q))/(C^2 - D^2 Q)`, a rational function and
+one beside the root, and each is answered
+([#718](https://github.com/asc-community/AngouriMath/issues/718)).
+
+The symbolic answers are large because each piece's answer is piecewise in the signs of the constants, and adding
+two of them multiplies their cases.
+
+| Input | Was (2.5.0) | Now |
+|---|---|---|
+| `"1/(a*c + b*c*x^2 + d*sqrt(a + b*x^2))".ToEntity().Integrate("x")` | `integral(...)` | 3,464 characters |
+| `"x^2/(a*c + b*c*x^2 + d*sqrt(a + b*x^2))".ToEntity().Integrate("x")` | `integral(...)` | 1,605 characters |
+| `"1/(x^2*(a*c + b*c*x^2 + d*sqrt(a + b*x^2)))".ToEntity().Integrate("x")` | `integral(...)` | 5,052 characters |
+| `"1/(c*sqrt(a + b*x^2) + d)".ToEntity().Integrate("x")` | `integral(...)` | 2,888 characters |
+| `"x*sqrt(2 - x^2)/(x - sqrt(2 - x^2))".ToEntity().Integrate("x")` | `integral(...)` | 195 characters |
+
 ### A quadratic with rational roots takes Euler's third substitution at one of them
 
 **Answers where there were none.** `1/(x + sqrt(3 - 2x - x^2))` was declined. Its leading coefficient is negative and

Sources/AngouriMath/Functions/Continuous/Integration/IndefiniteIntegralSolver.cs

@@ -18955,6 +18955,82 @@ node is Powf(var @base, Number.Rational r) && byRadicand.TryGetValue(@base, out
             return (rewritten, ratios);
         }
 
+        /// <summary>
+        /// A rational function of <c>x</c> and one square root of a quadratic <c>Q</c>, with the
+        /// root in a sum below the bar, written over a denominator free of it:
+        /// <c>(A + B sqrt(Q))/(C + D sqrt(Q))</c> is <c>(A + B sqrt(Q)) (C - D sqrt(Q))/(C^2 - D^2 Q)</c>,
+        /// a rational function and one beside the root, each asked of the chain.
+        /// </summary>
+        /// <remarks>
+        /// Rubi's <c>1/(a c + b c x^2 + d sqrt(a + b x^2))</c>: Euler's substitution takes the
+        /// generic first substitution for a symbolic leading coefficient, with <c>sqrt(b)</c> in
+        /// every coefficient, and was declined. Over the conjugate it is
+        /// <c>(c Q - d sqrt(Q))/(c^2 Q^2 - d^2 Q)</c>, and both pieces are answered. It runs after
+        /// every rule that answers the numeric spellings. Only a sum: a root alone below the bar,
+        /// <c>C = 0</c>, is the root moved above it, which the chain writes back, and <c>D = 0</c>
+        /// has nothing to clear.
+        /// https://github.com/asc-community/AngouriMath/issues/718
+        /// </remarks>
+        internal static Entity? SolveByTheConjugateOfASumWithARootOfAQuadraticBelowTheBar(Entity expr, Entity.Variable x, bool integrateByParts)
+        {
+            Entity? radicand = null;
+            foreach (var node in expr.Nodes)
+                if (node is Powf(var @base, Number.Rational r) && r is not Number.Integer && @base.ContainsNode(x))
+                {
+                    if (!r.ERational.Denominator.Equals(EInteger.FromInt32(2)) || !r.ERational.Numerator.CanFitInInt32()
+                        || radicand is not null && radicand != @base)
+                        return null;
+                    radicand = @base;
+                }
+            if (radicand is null || !TreeAnalyzer.TryGetPolyQuadratic(radicand, x, out var a, out _, out _) || VanishesIdentically(a))
+                return null;
+            var w = Variable.CreateUnique(expr, "w_root");
+            var inW = expr.Replace(node => node is Powf(var @base, Number.Rational r) && r is not Number.Integer && @base == radicand
+                ? MathS.Pow(w, Number.Integer.Create(r.ERational.Numerator))
+                : node);
+            var (top, bottom) = Functions.SingleQuotient.Of(Functions.SingleQuotient.Combine(inW));
+            if (!bottom.ContainsNode(w) || InTheRoot(top) is not var (above, aboveBeside) || InTheRoot(bottom) is not var (below, belowBeside)
+                || VanishesIdentically(below) || VanishesIdentically(belowBeside))
+                return null;
+            var over = (below * below - belowBeside * belowBeside * radicand).InnerSimplified;
+            if (VanishesIdentically(over))
+                return null;
+            var rational = (above * below - aboveBeside * belowBeside * radicand).InnerSimplified;
+            var beside = (aboveBeside * below - above * belowBeside).InnerSimplified;
+            Entity answer = Number.Integer.Zero;
+            if (!VanishesIdentically(rational))
+            {
+                if (Integration.ComputeIndefiniteIntegral(rational / over, x, integrateByParts) is not { } first)
+                    return null;
+                answer += first;
+            }
+            if (!VanishesIdentically(beside))
+            {
+                if (Integration.ComputeIndefiniteIntegral(beside * MathS.Sqrt(radicand) / over, x, integrateByParts) is not { } second)
+                    return null;
+                answer += second;
+            }
+            return answer;
+
+            // A polynomial in w as A + B w, with w^2 = Q.
+            (Entity Whole, Entity Beside)? InTheRoot(Entity side)
+            {
+                if (!TreeAnalyzer.TryGetPolynomial(side.Expand().InnerSimplified, w, out var monomials) || monomials.Keys.Any(k => k.Sign < 0))
+                    return null;
+                Entity whole = Number.Integer.Zero;
+                Entity besideTheRoot = Number.Integer.Zero;
+                foreach (var pair in monomials)
+                {
+                    // Q^0 written as a power is `1 provided not Q = 0`, a condition the pieces would carry.
+                    var k = pair.Key.ToInt32Checked();
+                    var term = k / 2 == 0 ? pair.Value : pair.Value * MathS.Pow(radicand, k / 2);
+                    if (k % 2 == 0) whole += term;
+                    else besideTheRoot += term;
+                }
+                return (whole.InnerSimplified, besideTheRoot.InnerSimplified);
+            }
+        }
+
         /// <summary>
         /// A rational function of <c>x</c> and one square root of a quadratic in <c>x</c>,
         /// rationalised by an Euler substitution and handed to the rational integrator.

Sources/AngouriMath/Functions/Continuous/Integration/Integration.Definition.cs

@@ -1197,6 +1197,9 @@ private static Entity Normalized(Entity expr, Entity.Variable x) =>
             // `A - B y` whose A/B agrees with a branch of y at the poles, found by an ansatz
             // over Padé approximants.
             if ((answer = SquareRootLogarithmAnsatz.Integrate(expr, x)) is { }) return answer;
+            // And a sum with the root below the bar, over its conjugate: after every rule above,
+            // which answer the numeric spellings, `1/(a c + b c x^2 + d sqrt(a + b x^2))` among them.
+            if ((answer = IndefiniteIntegralSolver.SolveByTheConjugateOfASumWithARootOfAQuadraticBelowTheBar(expr, x, integrateByParts)) is { }) return answer;
             // Product-to-sum among the rewrites rather than before them, because a product of
             // trigonometric functions of *equal* arguments is a power and wants a different tool;
             // this only fires where the arguments differ, which is exactly what every substitution

Sources/Tests/UnitTests/Calculus/TheConjugateOfASumWithARootBelowTheBarIntegralTest.cs

@@ -0,0 +1,78 @@
+//
+// Copyright (c) 2019-2026 Angouri.
+// AngouriMath is licensed under MIT.
+// Details: https://github.com/asc-community/AngouriMath/blob/master/LICENSE.md.
+// Website: https://am.angouri.org.
+//
+
+using System;
+using AngouriMath.Extensions;
+using Xunit;
+
+namespace AngouriMath.Tests.Calculus
+{
+    /// <summary>
+    /// A sum with the square root of a quadratic below the bar, cleared by its conjugate.
+    /// <a href="https://github.com/asc-community/AngouriMath/issues/718">#718</a>
+    /// </summary>
+    /// <remarks>
+    /// Rubi's <c>1/(a c + b c x^2 + d sqrt(a + b x^2))</c> and its <c>x^2</c> and <c>1/x^2</c>
+    /// neighbours had no antiderivative: Euler's substitution declines a symbolic leading
+    /// coefficient. Each answer is differentiated back with the constants pinned on both signs of
+    /// <c>b</c> and of <c>a c^2 - d^2</c>, at points where the root is real.
+    /// </remarks>
+    [Trait("Area", "Calculus")]
+    public sealed class TheConjugateOfASumWithARootBelowTheBarIntegralTest
+    {
+        private static void DifferentiatesBack(string integrand, double[] points, params (string, double)[] pins)
+        {
+            var integral = integrand.ToEntity().Integrate("x");
+            Assert.DoesNotContain("integral(", integral.Stringize());
+            Assert.DoesNotContain("NaN", integral.Stringize());
+
+            var derivative = integral.Substitute("C", 0).Differentiate("x");
+            Entity original = integrand.ToEntity();
+            foreach (var (name, value) in pins)
+            {
+                derivative = derivative.Substitute(name, value);
+                original = original.Substitute(name, value);
+            }
+
+            var compared = 0;
+            foreach (var at in points)
+            {
+                var got = derivative.Substitute("x", at).EvalNumerical();
+                var want = original.Substitute("x", at).EvalNumerical();
+                if (got.IsNaN || want.IsNaN)
+                    continue;
+                compared++;
+                var difference = Math.Abs((double)(got - want).RealPart)
+                               + Math.Abs((double)(got - want).ImaginaryPart);
+                var scale = Math.Max(1.0, Math.Abs((double)want.RealPart));
+                Assert.True(difference / scale < 1e-8,
+                    $"d/dx of the antiderivative of {integrand} is {got} at x = {at}, "
+                    + $"where the integrand is {want}");
+            }
+            Assert.True(compared >= 3,
+                $"only {compared} of {points.Length} points were comparable for {integrand}");
+        }
+
+        [Theory]
+        [InlineData("1/(a*c+b*c*x^2+d*sqrt(a+b*x^2))")]
+        [InlineData("x^2/(a*c+b*c*x^2+d*sqrt(a+b*x^2))")]
+        [InlineData("1/(x^2*(a*c+b*c*x^2+d*sqrt(a+b*x^2)))")]
+        [InlineData("1/(c*sqrt(a+b*x^2)+d)")]
+        public void SymbolicConstants(string integrand)
+        {
+            var points = new[] { 0.3, 0.7, 1.1, 1.3 };
+            DifferentiatesBack(integrand, points, ("a", 2), ("b", 3), ("c", 1.25), ("d", 1));
+            DifferentiatesBack(integrand, points, ("a", 2), ("b", 3), ("c", 0.5), ("d", 3));
+            DifferentiatesBack(integrand, points, ("a", 2), ("b", -1), ("c", 1), ("d", 0.5));
+            DifferentiatesBack(integrand, points, ("a", 2), ("b", 3), ("c", 1), ("d", -0.5));
+        }
+
+        [Fact]
+        public void ARootBesideXOverTheirDifference()
+            => DifferentiatesBack("x*sqrt(2-x^2)/(x-sqrt(2-x^2))", new[] { -0.5, 0.3, 1.2, 1.3 });
+    }
+}