Consult also Eric Weisstein 's page Pentagonal Octagonal Number . Sloane 's A048915 gives the first numbers that are both Pentagonal and Nonagonal . Consult also Eric Weisstein 's page Pentagonal Nonagonal Number . Believe it or not but some numbers can't be expressed as the sum of 3 pentagonal numbers. Not the sum of 3 pentagonal numbers.
Every pentagonal number is one-third of a triangular number.
Click on the image on the left for more background about the book. Click here to view some entries to the table about palindromes . He discovered those starting from index number 25 up to 35 .