A081931 Diagonal of triangle in A081930.

0, 22, 121, 1331, 10401, 105501, 1006001, 10077001, 100080001, 1000990001, 10001010001, 100011110001, 1000012100001, 10000133100001, 100000141000001, 1000001551000001, 10000001610000001, 100000017710000001, 1000000018100000001, 10000000199100000001, 100000000202000000001

Offset

1,2

Links

Table of n, a(n) for n=1..21.

Maple

isA002113 := proc(n) local digrev, d ; digrev := convert(n, base, 10) ; for d from 1 to nops(digrev)/2 do if op(d, digrev) <> op(-d, digrev) then RETURN(false) ; fi ; od ; RETURN(true) ; end: A081930 := proc(n) local a, k ; a := [] ; k := 10^(n-1) ; while nops(a) < n do if isA002113(k) then a := [op(a), k] ; fi ; k := k+1 ; od ; RETURN(a) ; end: A081931 := proc(n) local nrow ; nrow := A081930(n) ; RETURN(op(-1, nrow)) ; end: for n from 1 to 18 do A081931(n) ; od ; # R. J. Mathar, Apr 04 2007

Prog

(Python)
def A081931(n):
    s = str(10**((n-1)//2)-(n==1)+n-1)
    return int(s+s[-1-n%2::-1]) # Pontus von Brömssen, Aug 24 2024

Crossrefs

Cf. A081930, A081932.

Sequence in context: A039612 A085828 A221698 * A156293 A225308 A043498

Adjacent sequences: A081928 A081929 A081930 * A081932 A081933 A081934

Keyword

base,nonn

Author

Amarnath Murthy, Apr 01 2003

Extensions

More terms from R. J. Mathar, Apr 04 2007

More terms from Pontus von Brömssen, Aug 21 2024

a(1) corrected by Pontus von Brömssen, Aug 24 2024

Status

approved