Reference for Eruvin 112:7
רביע פלגא הוי אמר רבא בר אדא משוחאה אסברה לי משכחת לה במתא דהויא תרי אלפי אתרי אלפי תחום כמה הויא שיתסר קרנות כמה הויין שיתסר דל תמניא דתחומין וארבעה דקרנות כמה הוי תריסר
[And the open land,.] from the wall of the city and outward a thousand cubits round about,<span class="x" onmousemove="('comment',' Num. XXXV, 4, dealing with the cities of the Levites.');"><sup>21</sup></span> the Torah has thus enjoined, 'Surround the city by an open space of one thousand cubits'.' Such an open space [it was said] would represent a quarter of the entire area' - 'A quarter'! Is it not in fact one [in the neighbourhood] of a half?<span class="x" onmousemove="('comment',' One thousand cubits of open space in every two thousand cubits allowed: 1000/2000 = 1/2. The actual area of the open space on the present assumption would, of course, be less than a half of the total area, since an inner belt of the width of a thousand cubits is smaller in area than one of equal width around it.');"><sup>22</sup></span> - Raba replied: The surveyor Bar Adda<span class="x" onmousemove="('comment',' Aliter: The son of the surveyor Adda'. Aliter: 'Raba b. Adda replied: A surveyor.'');"><sup>23</sup></span>