A181558 Index of first occurrence of 2n in A031883, or 0 if 2n never occurs in A031883 = first differences of lucky numbers A000959.

1, 2, 6, 20, 31, 14, 126, 85, 36, 145, 140, 122, 376, 231, 162, 483, 692, 600, 993, 1188, 1106, 2440, 1080, 2814, 2586, 450, 2696, 3473, 4254, 4857, 5918, 2364, 4760, 4818, 21192, 13116, 19284, 14855, 12158, 31032, 18174, 15068, 35700, 56846, 27367, 33716, 13736, 16746, 103292, 8541

Offset: 1

Author

M. F. Hasler, Oct 31 2010

Keywords

• nonn

Examples

			a(1)=1 is the least index i such that A000959(i)+2*1 = A000959(i+1), since A000959(1) = 1 and A000959(2) = 3.
a(2)=2 is the least index i such that A000959(i)+2*2 = A000959(i+1), since A000959(2) = 3 and A000959(3) = 7.
a(3)=6 is the least index i such that A000959(i)+2*3 = A000959(i+1); indeed A000959(7) - A000959(6) = 21 - 15 is the earliest gap of 6 in A000959.
		

Crossrefs

Cf. A038664 (analog for primes).

Programs

PARI

a(n) = for( i=1,1e9, A031883[i]==2*n & return(i)) /* will issue an error if 2n is not found in A031883 */

Formula

a(n) = min { k | A031883(k)=2n } = min { k | A000959(k+1)=A000959(k)+2n }.

Extensions

a(20)-a(50) from Nathaniel Johnston, Nov 15 2010