A254636 Numbers that cannot be represented as x*y + x + y, where x>=y>1.

0, 1, 2, 3, 4, 5, 6, 7, 9, 10, 12, 13, 16, 18, 21, 22, 25, 28, 30, 33, 36, 37, 40, 42, 45, 46, 52, 57, 58, 60, 61, 66, 70, 72, 73, 78, 81, 82, 85, 88, 93, 96, 100, 102, 105, 106, 108, 112, 117, 121, 126, 130, 133, 136, 138, 141, 145, 148, 150, 156, 157, 162, 165, 166, 172

Offset

1,3

Comments

0, 7 and numbers n such that n+1 is either prime or twice a prime. - Robert Israel, Aug 05 2015

Links

Table of n, a(n) for n=1..65.

Maple

sort([0, 7, op(select(t -> isprime(t+1), [$1..10^4])), op(select(t -> isprime((t+1)/2), [2*i+1$i=1..5*10^3]))]); # Robert Israel, Aug 05 2015

Mathematica

r[n_] := Reduce[x >= y > 1 && n == x y + x + y, {x, y}, Integers];
Reap[For[n = 0, n <= 200, n++, If[r[n] === False, Sow[n]]]][[2, 1]] (* Jean-François Alcover, Feb 28 2019 *)

Crossrefs

Cf. A091529 (appears to be essentially the same, except first few terms).

Cf. A253975.

Sequence in context: A225804 A049809 A190278 * A165763 A178877 A011870

Adjacent sequences: A254633 A254634 A254635 * A254637 A254638 A254639

Keyword

nonn

Author

Alex Ratushnyak, Feb 03 2015

Status

approved