附录2. 本文章所用图片来源网址
<标题> https://www.freepik.com/free-photo/girls-hands-are-holding-cheesecake-made-berries-matcha-tea_23789675.htm#page=24&query=woman%20cooking%20cake&position=46&from_view=keyword&track=ais&uuid=0bc4af94-fa27-4994-918b-feca85bad846
<图1> 自制
<图2> http://wavefunction.fieldofscience.com/2017/04/kurt-godel-and-his-open-world.html
<图3> https://www.shapeways.com/product/WA5HL9UF3/grid-stereographic-projection
<图4> https://i.stack.imgur.com/xBEPh.gif
<图5> 自制
<图6> 自制
<图7> https://en.wikipedia.org/wiki/Apollo_8#/media/File:AS08-16-2593_remastered.jpg