This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A115709 #10 Dec 22 2021 07:03:13 %S A115709 12,22,51,176,330,532,590,715,782,925,1162,1247,1335,1426,1717,3151, %T A115709 3290,4187,5551,7107,7315,7957,10542,10795,11051,11837,12376,14950, %U A115709 15251,15555,15862,16172,17120,19097,19780,20126,22265,24130,24512,26467,26867,30175 %N A115709 Pentagonal numbers (A000326) whose digit reversal is a semiprime (A001358). %H A115709 Harvey P. Dale, <a href="/A115709/b115709.txt">Table of n, a(n) for n = 1..1000</a> %e A115709 532 is the 19th pentagonal number and 235=5*47 is semiprime. %t A115709 Select[PolygonalNumber[5,Range[200]],PrimeOmega[IntegerReverse[#]]==2&] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Jan 27 2019 *) %o A115709 (Python) %o A115709 from sympy import factorint %o A115709 def ok(p): return sum(e for e in factorint(int(str(p)[::-1])).values()) == 2 %o A115709 print([p for p in (n*(3*n-1)//2 for n in range(143)) if ok(p)]) # _Michael S. Branicky_, Dec 22 2021 %Y A115709 Cf. A000326, A001358, A115708. %K A115709 nonn,base %O A115709 1,1 %A A115709 _Giovanni Resta_, Jan 31 2006