Use the proper Unicode symbol for empty sets

core/src/main/scala/spire/math/Interval.scala

@@ -273,7 +273,7 @@ sealed abstract class Interval[A] extends Serializable { lhs =>
     case All() =>
       "(-∞, ∞)"
     case Empty() =>
-      "(Ø)"
+      "(∅)"
     case Above(lower, flags) =>
       if (isClosedLower(flags)) s"[$lower, ∞)" else s"($lower, ∞)"
     case Below(upper, flags) =>
@@ -949,7 +949,7 @@ object Interval {
   def atOrAbove[A: Order](a: A): Interval[A] = Above(a, 0)
   def atOrBelow[A: Order](a: A): Interval[A] = Below(a, 0)
 
-  private val NullRe = "^ *\\( *Ø *\\) *$".r
+  private val NullRe = "^ *\\( *∅ *\\) *$".r
   private val SingleRe = "^ *\\[ *([^,]+) *\\] *$".r
   private val PairRe = "^ *(\\[|\\() *(.+?) *, *(.+?) *(\\]|\\)) *$".r
 

core/src/main/scala/spire/math/interval/Overlap.scala

@@ -54,7 +54,7 @@ object Overlap {
 
   /**
    * [[inner]] is a subset of [[outer]]. Empty interval is always a subset of any other, so all overlaps on empty
-   * intervals go here, except `(Ø).overlap(Ø)`, that results in equality.
+   * intervals go here, except `(∅).overlap(∅)`, that results in equality.
    *
    * For example [1,4) and [1, 5]
    */

tests/shared/src/test/scala/spire/math/IntervalOverlapScalaCheckSuite.scala

@@ -36,7 +36,7 @@ class IntervalOverlapScalaCheckSuite extends munit.ScalaCheckSuite {
     }
   }
 
-  property("(x overlap Ø) = Subset(Ø, x) id x != Ø") {
+  property("(x overlap ∅) = Subset(∅, x) id x != ∅") {
     forAll { (x: Interval[Rational]) =>
       (x.nonEmpty) ==> {
         val empty = Interval.empty[Rational]

tests/shared/src/test/scala/spire/math/IntervalScalaCheckSuite.scala

@@ -47,7 +47,7 @@ class IntervalScalaCheckSuite extends munit.ScalaCheckSuite {
 
   val rng = spire.random.GlobalRng
 
-  property("(x -- y) ⊆ x && (x -- y) & y = Ø") {
+  property("(x -- y) ⊆ x && (x -- y) & y = ∅") {
     forAll { (x: Interval[Rational], y: Interval[Rational]) =>
       (x -- y).foreach { zi =>
         (zi.isSubsetOf(x)) &&
@@ -56,21 +56,21 @@ class IntervalScalaCheckSuite extends munit.ScalaCheckSuite {
     }
   }
 
-  property("(x -- Ø) = x") {
+  property("(x -- ∅) = x") {
     forAll { (x: Interval[Rational]) =>
       if (x.nonEmpty) {
         (x -- Interval.empty[Rational]) == List(x)
       } else true
     }
   }
 
-  property("(x -- x) = Ø") {
+  property("(x -- x) = ∅") {
     forAll { (x: Interval[Rational]) =>
       (x -- x) == Nil
     }
   }
 
-  property("(x -- (-∞, ∞)) = Ø") {
+  property("(x -- (-∞, ∞)) = ∅") {
     forAll { (x: Interval[Rational]) =>
       (x -- Interval.all[Rational]) == Nil
     }

tests/shared/src/test/scala/spire/math/IntervalSuite.scala

@@ -45,7 +45,7 @@ class IntervalSuite extends munit.FunSuite {
   test("Interval.closed(0, 5).isBounded is true") { assert(Interval.closed(0, 5).isBounded) }
 
   test("Interval.point(2).toString == [2]") { assertEquals(Interval.point(2).toString, "[2]") }
-  test("Interval.empty.toString == (Ø)") { assertEquals(Interval.empty[Int].toString, "(Ø)") }
+  test("Interval.empty.toString == (∅)") { assertEquals(Interval.empty[Int].toString, "(∅)") }
 
   val a = cc(0.0, 4.0)
   test("a.contains(0.0) is true") { assertEquals(a.contains(0.0), true) }