OFFSET
1,2
COMMENTS
Multiples of 11 plus or minus 1. If 11k+1 is a perfect square (see A219257) then a(n) is the square root of 11k+1. [Gary Detlefs, Feb 22 2010]
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
EXAMPLE
120 is in the sequence since |1-2+0| = 1.
MATHEMATICA
fQ[n_] := Abs[Plus @@ (((-1)^Range[Floor[Log[10, n] + 1]])*IntegerDigits@n)] == 1; Select[ Range@342, fQ@# &]
PROG
(PARI) altsum(v)=sum(i=1, #v, v[i]*(-1)^i)
is(n)=abs(altsum(digits(n)))==1 \\ Charles R Greathouse IV, May 21 2014
(Python)
def ok(n): return abs(sum(int(di)*(-1)**i for i, di in enumerate(str(n)))) == 1
print([k for k in range(343) if ok(k)]) # Michael S. Branicky, Jan 26 2023
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Robert G. Wilson v, May 10 2001
STATUS
approved