A078226 a(1) = 1, a(n+1) is the smallest odd multiple of a(n) (other than a(n) itself) in which the digits are alternately even and odd.

1, 3, 9, 27, 81, 567, 8505, 76545, 9874305, 6763898925, 41672381276925, 25432529276163496725, 6947294789656341278149816125, 2341412581496361870123890149638785410125

Offset

1,2

Links

Table of n, a(n) for n=1..14.

Example

a(6) = 567 = 7*a(5); the digits alternate odd, even, odd.

Maple

isA030141 := proc(n) local dgs, i ; dgs := convert(n, base, 10) ; for i from 1 to nops(dgs)-1 do if ( op(i, dgs)+op(i+1, dgs)) mod 2 = 0 then RETURN(false) ; fi ; od ; RETURN(true) ; end: A078226 := proc(nmax) local a, f; a := [1] ; while nops(a) < nmax do f := 3 ; while true do if isA030141(f*op(-1, a)) then a := [op(a), f*op(-1, a)] ; print(op(-1, a)) ; break ; fi ; f := f+2 ; od ; od ; end: A078226(13) ; # R. J. Mathar, Mar 01 2007

Prog

(Python)
A078226_list = [1]
for _ in range(20):
    x = A078226_list[-1]
    y, x2 = x, 2*x
    while True:
        y += x2
        s = str(y)
        for j in range(len(s)-1, -1, -2):
            if not s[j] in ('1', '3', '5', '7', '9'):
                break
        else:
            for k in range(len(s)-2, -1, -2):
                if not s[k] in ('0', '2', '4', '6', '8'):
                    break
            else:
                A078226_list.append(y)
                break
# Chai Wah Wu, Nov 06 2014

Crossrefs

Cf. A078221, A078222, A078223, A078224, A078225, A078227.

Sequence in context: A067500 A273898 A351343 * A083591 A052917 A099786

Adjacent sequences: A078223 A078224 A078225 * A078227 A078228 A078229

Keyword

base,nonn

Author

Amarnath Murthy, Nov 23 2002

Extensions

More terms from Sascha Kurz, Jan 30 2003

a(12) from R. J. Mathar, Mar 01 2007

a(13), a(14) from Max Alekseyev, May 12 2010

Status

approved