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 A030664 #29 Dec 18 2017 04:19:19 %S A030664 1,1,4,9,15,25,35,49,77,77,77,121,143,169,221,221,221,289,323,361,437, %T A030664 437,437,529,667,667,667,667,667,841,899,961,1147,1147,1147,1147,1147, %U A030664 1369,1517,1517,1517,1681,1763,1849,2021,2021,2021,2209,2491,2491,2491 %N A030664 Product of largest prime <= n and smallest prime >= n. %C A030664 Symmetrical about zero, a(n)=a(-n) if n>1, if negative primes are recognized. - _Robert G. Wilson v_, Feb 28 2011 %C A030664 Iff n is a prime then a(n)=n^2, otherwise a(n) is a semiprime. - _Robert G. Wilson v_, Feb 28 2011 %H A030664 T. D. Noe, <a href="/A030664/b030664.txt">Table of n, a(n) for n = 0..1000</a> %F A030664 For n > 1: a(n) = A007917(n)*A007918(n). - _Reinhard Zumkeller_, May 22 2015 %t A030664 f[n_] := If[Abs[n] < 2, 1, NextPrime[n + 1, -1] NextPrime[n - 1]]; Array[f, 51, 0] (* _Robert G. Wilson v_, Feb 28 2011 *) %o A030664 (MuPAD) numlib::prevprime(i)*nextprime(i) $ i = 0..50 // _Zerinvary Lajos_, Feb 26 2007 %o A030664 (Haskell) %o A030664 a030664 n = a007917 n * a007918 n -- _Reinhard Zumkeller_, May 22 2015, Feb 24 2011 %o A030664 (PARI) a(n) = if (n < 2, 1, precprime(n)*nextprime(n)); \\ _Michel Marcus_, Mar 21 2015 %Y A030664 Cf. A000040, A013638, A006094, A001248. %Y A030664 Cf. A007917, A007918, A013638. %K A030664 easy,nonn,nice %O A030664 0,3 %A A030664 _N. J. A. Sloane_ %E A030664 More terms from _Erich Friedman_