spectral-norm Swift program

/* The Computer Language Benchmarks Game
 https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
 contributed by Isaac Gouy
 converted to Swift 3 by Sergo Beruashvili
 */

import Glibc
//import Darwin
import Foundation

func approximate(_ n: Int) -> Double {
    var u = Array(repeating: 1.0, count: n)
    
    var v = Array(repeating: 0.0, count: n)
    for _ in 1...10 {
        multiplyAtAv(n,u,&v)
        multiplyAtAv(n,v,&u)
    }
    
    var vBv = 0.0, vv = 0.0
    for i in 0..<n {
        vBv += u[i]*v[i]
        vv  += v[i]*v[i]
    }
    
    return sqrt(vBv/vv)
}

func a(_ i: Int, _ j: Int) -> Double {
    let ij = i+j
    return 1.0 / Double( ij*(ij+1)/2 + i+1 )
}

func multiplyAv(_ n: Int, _ v: [Double], _ av: inout [Double]) {
    for i in 0..<n {
        av[i] = 0.0;
        for j in 0..<n {
            av[i] += a(i,j)*v[j]
        }
    }
}

func multiplyAtv(_ n: Int, _ v: [Double], _ atv: inout [Double]) {
    for i in 0..<n {
        atv[i] = 0;
        for j in 0..<n {
            atv[i] += a(j,i)*v[j]
        }
    }
}

func multiplyAtAv(_ n: Int, _ v: [Double], _ atAv: inout [Double]) {
    var u = Array(repeating: 0.0, count: n)
    multiplyAv(n,v,&u)
    multiplyAtv(n,u,&atAv)
}


let n: Int = Int(CommandLine.arguments[1])!
print( String(format: "%.9f", approximate(n)) )
    

NOTES:
64-bit Ubuntu quad core
Swift version 5.1.3 (swift-5.1.3-RELEASE)
Target: x86_64-unknown-linux-gnu



Tue, 05 May 2020 21:11:30 GMT

MAKE:
/opt/src/swift-5.1.3-RELEASE-ubuntu18.04/usr/bin/swiftc spectralnorm.swift -Ounchecked -target-cpu core2  -o spectralnorm.swift_run
<unknown>:0: warning: argument unused during compilation: '-mcpu=core2'

14.47s to complete and log all make actions

COMMAND LINE:
./spectralnorm.swift_run 5500

PROGRAM OUTPUT:
1.274224153