A255362 Numbers n such that neither n nor n+1 is representable as x*y+x+y, where x>=y>1.

0, 1, 2, 3, 4, 5, 6, 9, 12, 21, 36, 45, 57, 60, 72, 81, 105, 156, 165, 177, 192, 225, 261, 276, 312, 345, 357, 381, 396, 420, 456, 465, 477, 501, 540, 561, 585, 612, 660, 672, 717, 732, 756, 837, 861, 876, 885, 981, 996, 1017, 1092, 1152, 1185, 1200, 1212, 1236, 1281

Offset: 1

Author

Alex Ratushnyak, Feb 21 2015

Keywords

• nonn

Comments

Indices of double zeros in A255361.

Crossrefs

Cf. A255361.

Programs

PARI

iszero(n) = {for (y=2, n\2, for (x=y, n\2, if ((x*y+x+y) == n, return(0)););); return (1);}
isok(n) = iszero(n) && iszero(n+1); \\ Michel Marcus, Feb 22 2015

Python

TOP = 10000
a = [0]*TOP
for y in range(2, TOP//2):
  for x in range(y, TOP//2):
    k = x*y + x + y
    if k>=TOP: break
    a[k]+=1
print([n for n in range(TOP-1) if a[n]==0 and a[n+1]==0])