link_to_ar5iv_academus.js

// ==UserScript==
// @name     arxiv: add link to ar5iv and academ.us
// @version  1
// @grant    none
// @include  https://arxiv.org/abs/*
// ==/UserScript==

function add_link() {
  let url_ar5iv = document.URL.replace("arxiv","ar5iv");
  let li_ar5iv = document.createElement('li');
  li_ar5iv.innerHTML = '<a href="' + url_ar5iv + '">HTML (ar5iv)</a>'; 
  document.querySelector("#abs-outer .extra-services .full-text ul :nth-child(2)").after(li_ar5iv);
  
  let url_academus = "https://academ.us/search_result/?query=" + encodeURI(document.URL);
  let li_academus = document.createElement('li');
  li_academus.innerHTML = '<a href="' + url_academus + '">HTML (academ.us)</a>'; 
  li_ar5iv.after(li_academus);
};

window.addEventListener("load", add_link);