Integrate a trigonometric function of a + b/x beside a rational function in 1/x

BREAKING-CHANGES.md

@@ -396,6 +396,26 @@ in Rubi's 1.1.3.4 come out shorter as well, and those that took two seconds take
 | `"1/(x^8*(a + b*x^3)*(c + d*x^3))".ToEntity().Integrate("x")` | `integral(...)` | 1,217 characters |
 | `"1/(x^6*(a + b*x^4)*(c + d*x^4))".ToEntity().Integrate("x")` | `integral(...)` | 795 characters |
 
+### A trigonometric function of `a + b/x` beside a rational function is integrated in `1/x`
+
+**Answers where there were none.** `sin(c + d/x)/(e + f x)` was declined: the substitution search reads a candidate's
+derivative but never solves the candidate for `x`. Under `x = 1/u` it is `-sin(c + d u)/(u (e u + f)) du`, which the
+partial fractions and the sine and cosine integrals answer, and `x = 1/u` is one-to-one on either side of 0. Below a
+root of a linear, the linear radical substitution gets there first
+([#718](https://github.com/asc-community/AngouriMath/issues/718)).
+
+The answers below a root are large because the sine over the linears that come out of the substitution carries
+expanded coefficients ([#1877](https://github.com/asc-community/AngouriMath/issues/1877)).
+
+| Input | Was (2.5.0) | Now |
+|---|---|---|
+| `"sin(c + d/x)/(e + f*x)".ToEntity().Integrate("x")` | `integral(...)` | 196 characters |
+| `"(a + b*sin(c + d/x))/(e + f*x)^2".ToEntity().Integrate("x")` | `integral(...)` | 470 characters |
+| `"(a + b*sin(c + d/x))^2/(e + f*x)^3".ToEntity().Integrate("x")` | `integral(...)` | 2,516 characters |
+| `"(c*e + d*e*x)^(1/3)*sin(a + b/(c + d*x)^(1/3))".ToEntity().Integrate("x")` | `integral(...)` | 1,453 characters |
+| `"sin(a + b/sqrt(c + d*x))/(e + f*x)".ToEntity().Integrate("x")` | `integral(...)` | 12,010 characters |
+| `"sin(a + b/sqrt(c + d*x))/(e + f*x)^2".ToEntity().Integrate("x")` | `integral(...)` | 32,780 characters |
+
 ### A power of the cosine beside a secant binomial is integrated in the cosine
 
 **Answers where there were none.** `cos(c + d x)^2/(a + b sec(c + d x))` was declined, and so were the cosine's powers

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

@@ -19120,6 +19120,77 @@ Entity TakeOut(Entity node)
         /// <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 of trigonometric functions of <c>a + b/x</c>, under
+        /// <c>x = 1/u</c>: <c>sin(c + d/x)/(e + f x)</c> is
+        /// <c>-sin(c + d u)/(u (e u + f)) du</c>, which the partial fractions and the sine and
+        /// cosine integrals answer.
+        /// </summary>
+        /// <remarks>
+        /// <c>x = 1/u</c> is one-to-one on either side of 0, so the answer in <c>u</c> written back
+        /// in <c>1/x</c> holds on both. Every argument of a function holding <c>x</c> has to be
+        /// linear in <c>1/x</c> and not in <c>x</c>, so the integrand in <c>u</c> is one this rule
+        /// does not read again. Rubi's 4.1.12 <c>(a + b sin(c + d/x))^k/(e + f x)^m</c> were
+        /// declined: the substitution search reads a candidate's derivative but never solves
+        /// the candidate for <c>x</c>. https://github.com/asc-community/AngouriMath/issues/718
+        /// </remarks>
+        internal static Entity? SolveByTheReciprocalOfAFunctionOfTheReciprocal(Entity expr, Entity.Variable x, bool integrateByParts)
+        {
+            var u = Variable.CreateUnique(expr, "u_rec");
+            // Each function of x, named, and written in u: its argument the linear in u it is.
+            var inU = new Dictionary<Entity, Entity>();
+            var names = new Dictionary<Entity, Variable>();
+            foreach (var node in expr.Nodes)
+            {
+                Entity argument;
+                switch (node)
+                {
+                    case Sinf(var a): argument = a; break;
+                    case Cosf(var a): argument = a; break;
+                    case Tanf(var a): argument = a; break;
+                    case Cotanf(var a): argument = a; break;
+                    case Secantf(var a): argument = a; break;
+                    case Cosecantf(var a): argument = a; break;
+                    case Powf(_, Number.Integer):
+                    case Sumf or Minusf or Mulf or Divf or Variable or Number:
+                        continue;
+                    default:
+                        if (node.ContainsNode(x))
+                            return null;
+                        continue;
+                }
+                if (!argument.ContainsNode(x) || inU.ContainsKey(node))
+                    continue;
+                if (!TreeAnalyzer.TryGetPolynomial(argument.Substitute(x, Number.Integer.One / u).InnerSimplified, u, out var read)
+                    || read.Keys.Any(k => k.Sign < 0 || k.CompareTo(EInteger.One) > 0) || !read.TryGetValue(EInteger.One, out var slope))
+                    return null;
+                Entity linear = read.TryGetValue(EInteger.Zero, out var constant)
+                    ? constant.InnerSimplified + slope.InnerSimplified * u
+                    : slope.InnerSimplified * u;
+                inU[node] = node switch
+                {
+                    Sinf => MathS.Sin(linear),
+                    Cosf => MathS.Cos(linear),
+                    Tanf => MathS.Tan(linear),
+                    Cotanf => MathS.Cotan(linear),
+                    Secantf => MathS.Sec(linear),
+                    Cosecantf => MathS.Cosec(linear),
+                    _ => node,
+                };
+                names[node] = Variable.CreateUnique(expr, "f_rec" + names.Count);
+            }
+            if (inU.Count == 0)
+                return null;
+            // The rest one quotient, the way the rules read it: not `e + f/u` but `(e u + f)/u`.
+            var named = expr.Replace(node => names.TryGetValue(node, out var name) ? name : node);
+            var integrand = Functions.SingleQuotient.Combine(-named.Substitute(x, Number.Integer.One / u) / MathS.Sqr(u)).InnerSimplified;
+            foreach (var pair in names)
+                integrand = integrand.Substitute(pair.Value, inU[pair.Key]);
+            if (Integration.ComputeIndefiniteIntegral(integrand, u, integrateByParts) is not { } inTermsOfU)
+                return null;
+            return inTermsOfU.Substitute(u, Number.Integer.One / x);
+        }
+
         /// <summary>
         /// A root of a product beside a root of one of its factors, the product split:
         /// <c>(P Q)^r</c> is <c>K P^r Q^r</c>, where <c>K = (P Q)^r/(P^r Q^r)</c> is constant wherever

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

@@ -666,6 +666,9 @@ private static Entity Normalized(Entity expr, Entity.Variable x) =>
             if ((answer = IndefiniteIntegralSolver.SolveAGaussianInAReciprocal(expr, x)) is { }) return answer;
             // And a trigonometric function of the reciprocal of a linear beside a power of it, under u = 1/L.
             if ((answer = IndefiniteIntegralSolver.SolveAFunctionOfTheReciprocalOfALinear(expr, x, integrateByParts)) is { }) return answer;
+            // And one of `a + b/x` beside any rational function, under `x = 1/u`:
+            // `sin(c + d/x)/(e + f x)` is `-sin(c + d u)/(u (e u + f)) du`.
+            if ((answer = IndefiniteIntegralSolver.SolveByTheReciprocalOfAFunctionOfTheReciprocal(expr, x, integrateByParts)) is { }) return answer;
             // An exponential of a multiple of a logarithm is a power of the argument, which is
             // how every inverse hyperbolic function under an exponential arrives.
             if ((answer = IndefiniteIntegralSolver.SolveByFoldingAnExponentialOfALogarithm(expr, x, integrateByParts)) is { }) return answer;

Sources/Tests/UnitTests/Calculus/AFunctionOfTheReciprocalIntegralTest.cs

@@ -0,0 +1,79 @@
+//
+// 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 trigonometric function of <c>a + b/x</c> beside a rational function, under <c>x = 1/u</c>:
+    /// <c>sin(c + d/x)/(e + f x)</c> is <c>-sin(c + d u)/(u (e u + f)) du</c>.
+    /// <a href="https://github.com/asc-community/AngouriMath/issues/718">#718</a>
+    /// </summary>
+    /// <remarks>
+    /// Rubi's 4.1.12 <c>(a + b sin(c + d/x))^k/(e + f x)^m</c> were declined. Each answer is
+    /// differentiated back on both sides of 0.
+    /// </remarks>
+    [Trait("Area", "Calculus")]
+    public sealed class AFunctionOfTheReciprocalIntegralTest
+    {
+        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) + Math.Abs((double)want.ImaginaryPart));
+                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("sin(c+d/x)/(e+f*x)")]
+        [InlineData("cos(c+d/x)/(e+f*x)^2")]
+        [InlineData("(a+b*sin(c+d/x))/(e+f*x)^3")]
+        [InlineData("(a+b*sin(c+d/x))^2/(e+f*x)")]
+        public void AFunctionOfAPlusBOverX(string integrand)
+        {
+            var points = new[] { -1.4, -0.3, 0.8, 2.25 };
+            DifferentiatesBack(integrand, points, ("a", 1.0 / 3), ("b", 2), ("c", 1.5), ("d", 1.25), ("e", 2), ("f", 3));
+        }
+
+        [Theory]
+        [InlineData("(c*e+d*e*x)^(1/3)*sin(a+b/(c+d*x)^(1/3))")]
+        [InlineData("sin(a+b/sqrt(c+d*x))/(e+f*x)")]
+        public void AFunctionOfAPlusBOverARootOfALinear(string integrand)
+        {
+            var points = new[] { -0.3, 0.8, 2.25 };
+            DifferentiatesBack(integrand, points, ("a", 1.0 / 3), ("b", 2), ("c", 1.5), ("d", 1.25), ("e", 2), ("f", 3));
+        }
+    }
+}