Take a whole power out of a power of a product, in front of a constant

BREAKING-CHANGES.md

@@ -308,6 +308,26 @@ partial fractions divide. It is taken out in front of the integral first
 | `"(b*d + 2*c*d*x)^9/(a + b*x + c*x^2)^3".ToEntity().Integrate("x")` | past a minute | 76,249 characters, the length the content-free one already had |
 | `"(b*d + 2*c*d*x)^8/(a + b*x + c*x^2)^3".ToEntity().Integrate("x")` | past a minute | 65,236 characters |
 
+### A whole power is taken out of a power of a product, in front of a constant
+
+**Answers where there were none.** `(c sin(a + b x)^3)^(1/3)/x` was declined: nothing read a function under a root of
+its own power. `(c R f^n)^r` is `K f^(n r) R^r` with `K = (c R f^n)^r/(f^(n r) R^r)`, constant wherever it is defined,
+and only a factor of the integrand is rewritten. A constant stays under the root inside `K`, so a negative `c` is
+answered too: an odd root is the real root
+([#718](https://github.com/asc-community/AngouriMath/issues/718)).
+
+| Input | Was (2.5.0) | Now |
+|---|---|---|
+| `"(c*sin(a + b*x)^3)^(1/3)".ToEntity().Integrate("x")` | `integral(...)` | 35 characters |
+| `"(c*sin(a + b*x)^3)^(2/3)".ToEntity().Integrate("x")` | `integral(...)` | 78 characters |
+| `"(c*sin(a + b*x)^3)^(1/3)/x".ToEntity().Integrate("x")` | `integral(...)` | 105 characters |
+| `"(c*sin(a + b*x)^3)^(2/3)/x^3".ToEntity().Integrate("x")` | `integral(...)` | 391 characters |
+| `"(c*sin(a + b*x^n)^3)^(1/3)/x".ToEntity().Integrate("x")` | `integral(...)` | 129 characters |
+| `"sqrt((x^2 - 1)^2/(x*(1 + x^2)))/(1 + x^2)".ToEntity().Integrate("x")` | `integral(...)` | 167 characters |
+| `"1/sqrt((a^2 + x^2)^3)".ToEntity().Integrate("x")` | `integral(...)` | 104 characters |
+| `"1/sqrt((a + b*x^2)^3)".ToEntity().Integrate("x")` | `integral(...)` | 100 characters |
+| `"1/sqrt(1 - coth(x)^2)".ToEntity().Integrate("x")` | `integral(...)` | 301 characters |
+
 ### A power of x past the twelfth beside a block is split off
 
 **Answers where there were none.** `(c + d x^3 + g x^6 + f x^9)/(x^14 (a + b x^3))` was declined, and so were

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

@@ -19031,6 +19031,95 @@ node is Powf(var @base, Number.Rational r) && byRadicand.TryGetValue(@base, out
             }
         }
 
+        /// <summary>
+        /// A power of a product with a factor under a whole power, that power taken out:
+        /// <c>(c R f^n)^r</c> is <c>K f^(n r) R^r</c>, where <c>K = (c R f^n)^r/(f^(n r) R^r)</c> and
+        /// <c>c</c> is free of <c>x</c>. The logarithmic derivatives of both sides are
+        /// <c>r (R'/R + n f'/f)</c>, so <c>K</c> is
+        /// constant wherever it is defined, and it stands in front of the answer the way Rubi
+        /// writes <c>(c sin(a + b x)^3)^(1/3) csc(a + b x)</c> in front of its.
+        /// </summary>
+        /// <remarks>
+        /// <c>(c sin(a + b x)^3)^(1/3)/x</c>, <c>sqrt((x^2 - 1)^2/(x (1 + x^2)))/(1 + x^2)</c> and
+        /// <c>1/sqrt((a^2 + x^2)^3)</c> were declined: nothing reads a function under a root of its
+        /// own power. With the power taken out each is <c>K</c> times an integrand the chain
+        /// answers. After every other rule, and at the question asked only.
+        /// https://github.com/asc-community/AngouriMath/issues/718
+        /// </remarks>
+        internal static Entity? SolveByTakingTheWholePowersOutOfAPowerOfAProduct(Entity expr, Entity.Variable x, bool integrateByParts)
+        {
+            if (!Integration.AnsweringTheQuestionAsked || takingTheWholePowersOut)
+                return null;
+            var multiple = Variable.CreateUnique(expr, "k_out");
+            Entity? constant = null;
+            var rewritten = AFactor(expr);
+            if (constant is null)
+                return null;
+            var simplified = rewritten.InnerSimplified;
+            Entity? answer;
+            takingTheWholePowersOut = true;
+            try
+            {
+                answer = Integration.ComputeAsTheSameQuestion(simplified, x, integrateByParts);
+            }
+            finally
+            {
+                takingTheWholePowersOut = false;
+            }
+            if (answer is null)
+                return null;
+            answer = answer.Substitute(multiple, constant);
+            return answer.ContainsNode(multiple) ? null : answer;
+
+            // Only a factor of the integrand: inside a function, `ln(1/(c x) + sqrt(1/(c^2 x^2) - 1))`
+            // for `asech(c x)`, K is not a factor of anything, and the question it writes is no shorter.
+            Entity AFactor(Entity node) => node switch
+            {
+                Mulf(var left, var right) => AFactor(left) * AFactor(right),
+                Divf(var above, var below) => AFactor(above) / AFactor(below),
+                _ => TakeOut(node),
+            };
+
+            Entity TakeOut(Entity node)
+            {
+                if (constant is not null || node is not Powf(var @base, Number.Rational exponent) || exponent is Number.Integer
+                    || !@base.ContainsNode(x))
+                    return node;
+                var (above, below) = Functions.SingleQuotient.Of(Functions.SingleQuotient.Combine(@base));
+                Entity outside = Number.Integer.One;
+                Entity inside = Number.Integer.One;
+                var taken = false;
+                foreach (var (side, sign) in new[] { (above, 1), (below, -1) })
+                    foreach (var factor in Mulf.LinearChildren(side))
+                    {
+                        var (factorBase, factorExponent) = factor switch
+                        {
+                            Powf(var b, Number.Integer n) => (b, n),
+                            _ => (factor, Number.Integer.One),
+                        };
+                        if (!factorBase.ContainsNode(x))
+                            continue;
+                        if (factorExponent.EInteger.Abs().CompareTo(EInteger.FromInt32(2)) >= 0)
+                        {
+                            outside *= MathS.Pow(factorBase, (sign * factorExponent * exponent).InnerSimplified);
+                            taken = true;
+                        }
+                        else
+                            inside = sign > 0 ? inside * factor : inside / factor;
+                    }
+                if (!taken)
+                    return node;
+                // A factor free of x stays under the power, in K: taken out, `c^(1/3)` is only
+                // `c`'s cube root for `c > 0`.
+                var written = inside == Number.Integer.One ? outside : outside * MathS.Pow(inside, exponent);
+                constant = node / written;
+                return multiple * written;
+            }
+        }
+
+        /// <summary>Set while <see cref="SolveByTakingTheWholePowersOutOfAPowerOfAProduct"/> asks its question.</summary>
+        [System.ThreadStatic] private static bool takingTheWholePowersOut;
+
         /// <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.
@@ -28451,7 +28540,11 @@ Entity Rewritten(Entity side, Number.Integer sign)
                 if (constantAlone && (constant == Number.Integer.One || restFactors > 1
                     || !EntersOnlyThroughTrigonometricFunctions(single!, x)))
                     return node;
-                if (constant != Number.Integer.One && constant.Evaled is not Number.Real { IsPositive: true })
+                // An odd denominator is the real root, `(-8)^(1/3) = -2`, which splits across a
+                // product for any real constant: `(c sin(x)^3)^(1/3)` is `c^(1/3) sin(x)` for a
+                // negative `c` as well, and asking `c > 0` left it undefined there.
+                var aRealRoot = exponent is Number.Rational { ERational: var fraction } && !fraction.Denominator.IsEven;
+                if (constant != Number.Integer.One && constant.Evaled is not Number.Real { IsPositive: true } && !aRealRoot)
                 {
                     if (constant.Vars.Any() is false)
                         return node;

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

@@ -1200,6 +1200,9 @@ private static Entity Normalized(Entity expr, Entity.Variable x) =>
             // 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;
+            // And a power of a product with a factor under a whole power, that power taken out in
+            // front of a constant: `(c sin(a + b x)^3)^(1/3)` is `K sin(a + b x)`.
+            if ((answer = IndefiniteIntegralSolver.SolveByTakingTheWholePowersOutOfAPowerOfAProduct(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/AWholePowerTakenOutOfAPowerOfAProductIntegralTest.cs

@@ -0,0 +1,80 @@
+//
+// 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 power of a product with a factor under a whole power, that power taken out in front of
+    /// a constant: <c>(c sin(a + b x)^3)^(1/3)</c> is <c>K sin(a + b x)</c>.
+    /// <a href="https://github.com/asc-community/AngouriMath/issues/718">#718</a>
+    /// </summary>
+    /// <remarks>
+    /// Rubi's 4.1.12 and 1.3.2 were declined. Each answer is differentiated back with the constant
+    /// under the root on both signs: an odd root is the real root, and a negative <c>c</c> used to
+    /// be answered <c>provided c &gt; 0</c>.
+    /// </remarks>
+    [Trait("Area", "Calculus")]
+    public sealed class AWholePowerTakenOutOfAPowerOfAProductIntegralTest
+    {
+        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("(c*sin(a+b*x)^3)^(1/3)")]
+        [InlineData("(c*sin(a+b*x)^3)^(2/3)")]
+        [InlineData("(c*sin(a+b*x)^3)^(1/3)/x")]
+        [InlineData("(c*sin(a+b*x)^3)^(2/3)/x^3")]
+        [InlineData("(c*sin(a+b*x^2)^3)^(1/3)/x^3")]
+        public void AConstantOfEitherSignUnderAnOddRoot(string integrand)
+        {
+            var points = new[] { -1.4, -0.3, 0.8, 2.25 };
+            DifferentiatesBack(integrand, points, ("a", 1.0 / 3), ("b", 2), ("c", 2));
+            DifferentiatesBack(integrand, points, ("a", 1.0 / 3), ("b", 2), ("c", -3));
+        }
+
+        [Theory]
+        [InlineData("sqrt((-1+x^2)^2/(x*(1+x^2)))/(1+x^2)", new[] { 0.5, 0.8, 2.0, 3.0 })]
+        [InlineData("1/sqrt((a^2+x^2)^3)", new[] { -2.0, 0.3, 2.5 })]
+        [InlineData("1/sqrt((a+b*x^2)^3)", new[] { -1.3, 0.3, 1.1 })]
+        public void ARootOfAPowerOfAPolynomial(string integrand, double[] points)
+            => DifferentiatesBack(integrand, points, ("a", 2), ("b", -1));
+    }
+}