Using copy_n, C++ (gcc)

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//Copy

#include <iostream>
#include <list>
#include <algorithm>
#include <cmath>

using namespace std;

template<class T>T value(const T& x){
    return pow(x, 2) + 3*x + 1;
};

int main()
{
    const int sz1 = 15;
    const int sz2 = 20;
    const int sz3 = 30;
    
    list<int>L1;
    list<int>L2;
    list<int>L3;
    
    int x = 0;
    int y = 0;
    for(int i = 0; i < sz1; i++){
        y = value(x);
        
        L1.push_front(y);
        
        ++x;
    }
    
    cout << "\nL1\t[ ";    
    for(auto i : L1)
        cout << i << "  ";
    cout << "]\n";
    
    L2.assign(L1.begin(), L1.end());
    cout << "L2:\t[ ";
    for(auto i : L2)
        cout << i << "  ";
    cout << "]\n";
    
    //std::copy_n
    //
    copy_n(L1.begin(), 15, inserter(L3, L3.begin()));
    
    cout << "L3:\t[ ";
    for(auto i : L3)
        cout << i << "  ";
    cout << "]\n";
           
    // Adjust to the subsequence size
    //
    int n = 10; // number of elements to copy (subswquence size)
    list<int>L4;
    
    (n > L1.size()) ? (n = L1.size()) : n; // must not go beyond the size of the source
    
    int val = value(n-1); // value at n-1
    
    list<int>::iterator LIter1;
    LIter1 = find(L1.begin(), L1.end(), val); // use the algorithm 'find' to locate the value 'val' in list
    
    copy_n(LIter1, n, inserter(L4, L4.begin())); // use the iterator as the start point of sequence to copy          
    
    cout << "\nvalue = " << val << "\tnumber of elements = " << n << "\n\n";        
    cout << "L4:\t[ ";
    for(auto i : L4)
        cout << i << "  ";
    cout << "]\n";

}

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Compilation time: 1.03 sec, absolute running time: 0.14 sec, cpu time: 0 sec, memory peak: 3 Mb, absolute service time: 1.18 sec