A175500 a(1) = 1. a(n) = the smallest integer not yet occurring such that if d(a(n)) = d(a(k)), then d(a(n-1)) doesn't equal d(a(k-1)) for any k where 2<= k <= n-1, where d(m) = the number of divisors of m.
1, 2, 3, 4, 5, 6, 7, 12, 8, 9, 10, 14, 16, 11, 24, 13, 36, 15, 18, 17, 48, 19, 60, 20, 25, 28, 30, 21, 40, 32, 44, 64, 22, 72, 23, 81, 26, 80, 27, 100, 29, 120, 31, 144, 33, 168, 34, 180, 35, 192, 37, 240, 38, 324, 41, 252, 42, 49, 54, 56, 84, 39, 336, 43
Offset: 1
Links
- Paul Tek, Table of n, a(n) for n = 1..2473
- Paul Tek, C++ program for this sequence
Programs
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PARI
ok(j, va, vs, n) = {if (vecsearch(vs, j), return (0)); for (k=1, n-1, if ((numdiv(j) == numdiv(va[k])) && (numdiv(va[k-1]) == numdiv(va[n-1])), return (0));); 1;} findnew(va, vs, n) = {my(j = 1); my(vs = vecsort(va)); until (ok(j, va, vs, n), j++); j;} lista(nn) = {my(va = [1]); for (n=2, nn, vs = vecsort(va); newa = findnew(va, vs, n); va = concat(va, newa);); va;} \\ Michel Marcus, May 04 2016
Extensions
a(26)-a(64) from Paul Tek, Mar 05 2014
Comments