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 A046429 #14 Oct 16 2019 06:17:54 %S A046429 40,44,81,224,265,395,422,462,640,698,818,972,1010,1032,1070,1089, %T A046429 1174,1206,1280,1336,1446,1518,1520,1528,1581,1662,1728,1814,1816, %U A046429 1849,1852,1853,1856,1892,1927,1932,1960,2032,2060,2061,2090,2098,2202,2212,2249 %N A046429 Numbers requiring 9 steps to reach a prime under the prime factor concatenation procedure. %H A046429 David A. Corneth, <a href="/A046429/b046429.txt">Table of n, a(n) for n = 1..10000</a> %H A046429 Patrick De Geest, <a href="http://www.worldofnumbers.com/topic1.htm">Home Primes</a> %e A046429 698 is in the sequence as 698 -> 2349 -> 333329 -> 2571297 -> 3857099 -> 31312323 -> 33771937101 -> 379437170413 -> 73124171910091 -> 374148203145623. Only after the ninth iteration we reach a prime. - _David A. Corneth_, Oct 15 2019 %o A046429 (PARI) is(n, k) = if(isprime(n), return(0)); for(i = 1, k - 1, n = concatelements(primesvector(n)); if(isprime(n), return(0))); n = concatelements(primesvector(n)); isprime(n) %o A046429 concatelements(v) = my(s = ""); for(i = 1, #v, s = concat(s, v[i])); eval(s) %o A046429 primesvector(n) = my(f = factor(n), res = vector(vecsum(f[,2])), t = 0); for(i = 1, #f~, for(j = 1, f[i, 2], t++; res[t] = f[i, 1])); res \\ _David A. Corneth_, Oct 15 2019 %Y A046429 Cf. A037271, A046411. %K A046429 nonn,base %O A046429 1,1 %A A046429 _Patrick De Geest_, Jun 15 1998 %E A046429 Extended and edited by _Charles R Greathouse IV_, Apr 28 2010