What a reduction leaves over its block is integrated in named coefficients

BREAKING-CHANGES.md

@@ -253,6 +253,30 @@ then cancels before the division of the improper fraction
 | `"(a + b*x)^6/(a*c + (b*c + a*d)*x + b*d*x^2)^2".ToEntity().Integrate("x")` | past a minute | 3,592 characters |
 | `"csc(e + f*x)^3/(a + b*tan(e + f*x)^2)^2".ToEntity().Integrate("x")` | `integral(...)` | 7,992 characters |
 
+### What a reduction leaves over its block is integrated in named coefficients
+
+**Answers where there were none, and sooner.** `(c + d x^3 + g x^6 + f x^9)/(x^4 (a + b x^3)^3)` took 42 s on the
+unreleased master, and `x^5`, `x^7` and `x^8` in place of `x^4` took 35 to 50 s, Rubi's 1.1.3.8. Four polynomials
+over `(a + b x^2 + c x^4)^2` and `^3` in 1.2.2.5 ran past two minutes, and
+`(d + f x + g x^2 + h x^3 + k x^4)/(a + b x^2 + c x^4)^3` took 31 s. Two reductions -- the split at a power of x beside
+a block, and the one that takes a polynomial over a power of a trinomial in `x^n` down to its first power --
+integrated what was left over the block with the coefficients they had computed, such as
+`(a^2 g - 3 a b d + 6 b^2 c)/a^2`, and the substitution search spent up to a minute simplifying quotients in them
+before another rule answered. Each such coefficient is a fresh symbol while the rest is integrated now, and is put
+back in the answer. The answers over a power of the trinomial are large, 22,000 to 98,000 characters, and right.
+One answer grew: `1/(x^7 (a + b x^3)(c + d x^3))` took 52 s on the unreleased master for 14,565 characters, and
+takes 6 s for 265,499 now ([#718](https://github.com/asc-community/AngouriMath/issues/718)).
+
+| Input | Was (2.5.0) | Now |
+|---|---|---|
+| `"(c + d*x^3 + g*x^6 + f*x^9)/(x^4*(a + b*x^3)^3)".ToEntity().Integrate("x")` | `integral(...)` | 547 characters |
+| `"(c + d*x^3 + g*x^6 + f*x^9)/(x^8*(a + b*x^3)^3)".ToEntity().Integrate("x")` | `integral(...)` | 1,859 characters |
+| `"1/(x^7*(a + b*x^3)*(c + d*x^3))".ToEntity().Integrate("x")` | `integral(...)` | 265,499 characters |
+| `"(d + f*x + g*x^2 + h*x^3 + k*x^4)/(a + b*x^2 + c*x^4)^3".ToEntity().Integrate("x")` | `integral(...)` | 31,531 characters |
+| `"(d + f*x + g*x^2 + h*x^3 + k*x^4 + m*x^5 + n*x^6 + p*x^7)/(a + b*x^2 + c*x^4)^2".ToEntity().Integrate("x")` | `integral(...)` | 21,719 characters |
+| `"(d + f*x + g*x^2 + h*x^3 + k*x^4 + m*x^5 + n*x^8 + p*x^11)/(a + b*x^2 + c*x^4)^3".ToEntity().Integrate("x")` | `integral(...)` | 77,389 characters |
+| `"csc(c + d*x)^2/(a - b*sin(c + d*x)^4)^3".ToEntity().Integrate("x")` | `integral(...)` | 157,787 characters |
+
 ### One linear twice is not two linear powers
 
 **Answers that had no value.** `(a c + b c x)^(-3 - 2 p) (f + g x) (a^2 + 2 a b x + b^2 x^2)^p` was answered

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

@@ -1361,15 +1361,15 @@ Entity Coefficient(Dictionary<EInteger, Entity> polynomial, int power)
             }
 
             // R = (N - P B)/x^k, whose coefficient of x^(m - k) is that of x^m in N - P B.
-            Entity remainder = Number.Integer.Zero;
+            var remainder = new Dictionary<int, Entity>();
             for (var m = k; m <= System.Math.Max(degreeAbove, degreeBelow + k - 1); m++)
             {
                 var coefficient = Coefficient(above, m);
                 for (var j = System.Math.Max(0, m - degreeBelow); j <= System.Math.Min(k - 1, m); j++)
                     coefficient -= series[j] * Coefficient(below, m - j);
                 coefficient = Functions.PartialFractions.InLowestTermsOverTheSymbols(coefficient);
                 if (!VanishesIdentically(coefficient))
-                    remainder += m == k ? coefficient : coefficient * MathS.Pow(x, m - k);
+                    remainder[m - k] = coefficient;
             }
 
             // The part over x^k term by term, and the rest over the block as it is written.
@@ -1379,7 +1379,7 @@ Entity Coefficient(Dictionary<EInteger, Entity> polynomial, int power)
                     overX += j == k - 1
                         ? series[j] * IntegralPatterns.AntiderivativeLog(x)
                         : series[j] * MathS.Pow(x, j - k + 1) / (j - k + 1);
-            if (remainder == Number.Integer.Zero)
+            if (remainder.Count == 0)
                 return overX;
             return OverTheRestOfTheBlock(remainder, block, x, integrateByParts) is { } overTheBlock
                 ? overX + overTheBlock
@@ -1398,10 +1398,69 @@ Entity Coefficient(Dictionary<EInteger, Entity> polynomial, int power)
         /// https://github.com/asc-community/AngouriMath/issues/718
         /// </remarks>
         private static Entity? OverTheRestOfTheBlock(Entity remainder, Entity block, Entity.Variable x, bool integrateByParts)
+            => AtAnotherLinearOfTheBlock(remainder, block, x, integrateByParts)
+               ?? Integration.ComputeIndefiniteIntegral(remainder / block, x, integrateByParts);
+
+        /// <summary>
+        /// <see cref="OverTheRestOfTheBlock(Entity, Entity, Entity.Variable, bool)"/> for a rest
+        /// given by its coefficients, integrated a level down in named coefficients.
+        /// </summary>
+        private static Entity? OverTheRestOfTheBlock(Dictionary<int, Entity> rest, Entity block, Entity.Variable x, bool integrateByParts)
+        {
+            Entity remainder = Number.Integer.Zero;
+            foreach (var pair in rest.OrderBy(pair => pair.Key))
+                remainder += pair.Key == 0 ? pair.Value : pair.Value * (pair.Key == 1 ? x : MathS.Pow(x, pair.Key));
+            return AtAnotherLinearOfTheBlock(remainder, block, x, integrateByParts)
+                ?? IntegratedInNamedCoefficients(rest, block, x, integrateByParts);
+        }
+
+        /// <summary>
+        /// <c>R/B</c> split at a linear the block still holds, where that answer differentiates back.
+        /// </summary>
+        private static Entity? AtAnotherLinearOfTheBlock(Entity remainder, Entity block, Entity.Variable x, bool integrateByParts)
             => IntegrateOverAPowerOfALinearBesideABlock(remainder, block, x, integrateByParts) is { } atAnotherRoot
                && Functions.PartialFractions.DerivativeHoldsAtSampledPoints(atAnotherRoot, remainder / block, x)
                 ? atAnotherRoot
-                : Integration.ComputeIndefiniteIntegral(remainder / block, x, integrateByParts);
+                : null;
+
+        /// <summary>
+        /// The integral of <c>R/B</c>, for a polynomial <c>R</c> in x given by its coefficients, with each coefficient of
+        /// <c>R</c> that is more than a symbol or a number a fresh symbol while it is integrated,
+        /// and put back in the answer.
+        /// </summary>
+        /// <remarks>
+        /// What a reduction leaves over its block has coefficients the reduction computed, such as
+        /// <c>(a^2 g - 3 a b d + 6 b^2 c)/a^2</c>, and the substitution search simplifies each
+        /// quotient it tries in them. Over <c>(a/b + x^3)^3</c> that took 40 s to decline where three
+        /// symbols take it 45 ms, and over <c>(a + b x^2 + c x^4)</c> past a minute.
+        /// https://github.com/asc-community/AngouriMath/issues/718
+        /// </remarks>
+        private static Entity? IntegratedInNamedCoefficients(Dictionary<int, Entity> rest, Entity block, Entity.Variable x, bool integrateByParts)
+        {
+            var named = new List<(Variable Name, Entity Value)>();
+            Entity everything = block;
+            foreach (var value in rest.Values)
+                everything += value;
+            Entity inNames = Number.Integer.Zero;
+            foreach (var pair in rest.OrderBy(pair => pair.Key))
+            {
+                var power = pair.Key;
+                var coefficient = pair.Value;
+                if (coefficient is not (Variable or Number))
+                {
+                    var name = Variable.CreateUnique(everything, "k_r");
+                    everything += name;
+                    named.Add((name, coefficient));
+                    coefficient = name;
+                }
+                inNames += power == 0 ? coefficient : coefficient * (power == 1 ? x : MathS.Pow(x, power));
+            }
+            if (Integration.ComputeIndefiniteIntegral(inNames / block, x, integrateByParts) is not { } answer)
+                return null;
+            foreach (var (name, value) in named)
+                answer = answer.Substitute(name, value);
+            return answer;
+        }
 
         /// <summary>
         /// <c>N/(L^k B)</c> for a linear <c>L = g + h x</c> other than x, beside a block <c>B</c>
@@ -1897,13 +1956,13 @@ Entity InU(Dictionary<EInteger, Entity> polynomial)
             Entity answer = Number.Integer.Zero;
             foreach (var pair in overPowers.OrderByDescending(pair => pair.Key))
                 answer = answer + pair.Value / (pair.Key == 1 ? trinomial : MathS.Pow(trinomial, pair.Key));
-            Entity rest = Number.Integer.Zero;
-            foreach (var pair in atThisPower.OrderBy(pair => pair.Key))
+            var rest = new Dictionary<int, Entity>();
+            foreach (var pair in atThisPower)
                 if (!VanishesIdentically(pair.Value))
-                    rest = rest + pair.Value * (pair.Key == 0 ? Number.Integer.One : pair.Key == 1 ? x : MathS.Pow(x, pair.Key));
-            if (rest != Number.Integer.Zero)
+                    rest[pair.Key] = pair.Value;
+            if (rest.Count > 0)
             {
-                if (Integration.ComputeIndefiniteIntegral(rest / trinomial, x, integrateByParts) is not { } overTheTrinomial)
+                if (IntegratedInNamedCoefficients(rest, trinomial, x, integrateByParts) is not { } overTheTrinomial)
                     return null;
                 answer = answer + overTheTrinomial;
             }

Sources/Tests/UnitTests/Calculus/TheRestOverABlockInNamedCoefficientsIntegralTest.cs

@@ -0,0 +1,45 @@
+//
+// 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>
+    /// What a reduction leaves over its block is integrated with its compound coefficients named:
+    /// with them as written, the substitution search spent up to a minute declining it, and these
+    /// did not finish. Rubi's 1.1.3.8 and 1.2.2.5.
+    /// <a href="https://github.com/asc-community/AngouriMath/issues/718">#718</a>
+    /// </summary>
+    [Trait("Area", "Calculus")]
+    public sealed class TheRestOverABlockInNamedCoefficientsIntegralTest
+    {
+        [Theory]
+        [InlineData("(c + d*x^3 + g*x^6 + f*x^9)/(x^4*(a + b*x^3)^3)")]
+        [InlineData("(c + d*x^3 + g*x^6 + f*x^9)/(x^8*(a + b*x^3)^3)")]
+        [InlineData("(d*x + g*x^3)/(a + b*x^2 + c*x^4)^3")]
+        public void DifferentiatesBack(string integrand)
+        {
+            var integral = integrand.ToEntity().Integrate("x");
+            Assert.DoesNotContain("integral(", integral.Stringize());
+            Entity Pinned(Entity e) => e.Substitute("a", 2).Substitute("b", 3).Substitute("c", 5)
+                .Substitute("d", -1).Substitute("g", 7).Substitute("f", 1);
+            var derivative = Pinned(integral.Substitute("C", 0)).Differentiate("x");
+            var original = Pinned(integrand.ToEntity());
+            foreach (var at in new[] { -1.7, -0.6, 0.45, 1.3 })
+            {
+                var want = original.Substitute("x", at).EvalNumerical();
+                var got = derivative.Substitute("x", at).EvalNumerical();
+                Assert.True(Math.Abs((double)(got - want).RealPart) + Math.Abs((double)(got - want).ImaginaryPart)
+                            < 1e-9 * Math.Max(1, Math.Abs((double)want.RealPart)),
+                    $"d/dx of the antiderivative of {integrand} is {got} at x = {at}, where the integrand is {want}");
+            }
+        }
+    }
+}