A066200 a(1) = 1; for n > 1, a(n) = a(n-1)-(n+2) if this is positive and has not already appeared, otherwise a(n) = a(n-1)+(n+2).

1, 5, 10, 4, 11, 3, 12, 2, 13, 25, 38, 24, 9, 25, 8, 26, 7, 27, 6, 28, 51, 75, 50, 76, 49, 21, 50, 20, 51, 19, 52, 18, 53, 17, 54, 16, 55, 15, 56, 14, 57, 101, 146, 100, 147, 99, 148, 98, 47, 99, 46, 100, 45, 101, 44, 102, 43, 103, 42, 104, 41, 105, 40, 106, 39, 107, 176

Offset: 1

Author

N. J. A. Sloane, Dec 16 2001

Keywords

• nonn
• easy
• look

Links

• Robert Israel, Table of n, a(n) for n = 1..10000
• Index entries for sequences related to Recamán's sequence

Crossrefs

A variant of A005132. A row of the array in A066201.

Programs

Maple

S:= {1}:
a[1]:= 1:
for n from 2 to 100 do
  t:=a[n-1]-(n+2);
  if t > 0 and not member(t,S) then
    a[n]:= t
  else
    a[n]:= a[n-1]+(n+2)
  fi;
  S:= S union {a[n]}
od:
seq(a[n],n=1..100); # Robert Israel, Feb 10 2017

Python

l=[0, 1]
for n in range(2, 101):
    x=l[n - 1] - (n + 2)
    if x>0 and x not in l: l.append(x)
    else: l.append(l[n - 1] + (n + 2))
print(l[1:]) # Indranil Ghosh, Jun 02 2017

Extensions

More terms from Vladeta Jovovic, Dec 16 2001