A128204 a(0) = 0; a(n) = a(n-1) - (2n-1) if that number is positive and not already in the sequence, otherwise a(n) = a(n-1) + (2n-1).

0, 1, 4, 9, 2, 11, 22, 35, 20, 3, 22, 43, 66, 41, 14, 43, 12, 45, 10, 47, 8, 49, 6, 51, 98, 147, 96, 149, 94, 37, 96, 157, 220, 155, 88, 19, 90, 17, 92, 15, 94, 13, 96, 181, 268, 179, 270, 177, 82, 179, 80, 181, 78, 183, 76, 185, 74, 187, 72, 189, 70, 191, 68, 193, 320

Offset: 0

Author

Nick Hobson, Feb 19 2007

Keywords

• easy
• nonn

Comments

'Recamán transform' (see A005132) of the odd numbers.

Examples

			Consider n=7. We have a(6)=22 and try to subtract 13, the 7th odd number. The result, 9, is certainly positive, but we cannot use it because 9 is already in the sequence. So we must add 13 instead, getting a(7) = 22 + 13 = 35.
		

Links

• Nick Hobson, Python program for this sequence
• Index entries for sequences related to Recamán's sequence

Crossrefs

Cf. A005132, A053461, A064365, A123483.

Programs

PARI

A128204(N,s/*=1 to print all terms*/)={my(a=0,u=0);  for( n=1,N, s&print1(a","); u=bitor(u,2^a+=if(a<2*n || bittest(u,a+1-2*n), 2*n-1,1-2*n)));a} \\ M. F. Hasler, Mar 07 2012