A231964 - OEIS

A231964

Lesser member of untouchable twin pairs, that is, such that a(n) and a(n)+2 are untouchable (A005114).

%I #21 Jun 13 2021 03:23:17

%S 246,288,290,304,322,324,406,472,516,518,624,626,668,748,766,782,802,

%T 892,894,896,934,964,1044,1078,1146,1148,1160,1246,1254,1256,1312,

%U 1314,1316,1346,1404,1418,1420,1506,1508,1680,1716,1774,1820,1838,1840,1842,1894

%N Lesser member of untouchable twin pairs, that is, such that a(n) and a(n)+2 are untouchable (A005114).

%C There are 2673 terms in this sequence when the 8153 terms of A005114, up to 60000, are taken into account.

%C Note that some numbers come up by pairs like (288,290) or (322,324), corresponding to untouchable triples. Others come up by groups of 3 like (892,894,896) or (1312,1314,1316), corresponding to untouchable quadruplets. For higher n-tuplets, see A231965.

%H Donovan Johnson, <a href="/A231964/b231964.txt">Table of n, a(n) for n = 1..10000</a>

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/UntouchableNumber.html">Untouchable Number.</a>

%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Untouchable_number">Untouchable number</a>

%e 246 and 248 are untouchable, so 246 is in this sequence.

%o (PARI) lista() = {v = readvec("untouchable.log"); for (i=1, #v, vi = v[i]; if (vecsearch(v, vi+2, ), print1(vi, ", ")););} \\ _Michel Marcus_, Nov 16 2013

%Y Cf. A005114, A231965.

%K nonn

%O 1,1

%A _Michel Marcus_, Nov 16 2013