A061870 Numbers such that |first digit - second digit + third digit - fourth digit ...| = 1.

1, 10, 12, 21, 23, 32, 34, 43, 45, 54, 56, 65, 67, 76, 78, 87, 89, 98, 100, 111, 120, 122, 131, 133, 142, 144, 153, 155, 164, 166, 175, 177, 186, 188, 197, 199, 210, 221, 230, 232, 241, 243, 252, 254, 263, 265, 274, 276, 285, 287, 296, 298, 320, 331, 340, 342

Offset: 1

Author

Robert G. Wilson v, May 10 2001

Keywords

• nonn
• base
• easy

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]

Examples

			120 is in the sequence since |1-2+0| = 1.
		

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A008593, A060978-A060980, A060982, A061470-A061479, A061870-A061882.

Subsequence of A175885.

Programs

Mathematica

fQ[n_] := Abs[Plus @@ (((-1)^Range[Floor[Log[10, n] + 1]])*IntegerDigits@n)] == 1; Select[ Range@342, fQ@# &]

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