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A306384 Start with n and repeatedly double it and apply the "delete any run of identical digits" operation described in A321801; a(n) is the number of steps needed to reach one of 0, 1, or 5, or -1 if none of these three numbers is ever reached. 1
0, 0, 19, 12, 18, 0, 11, 23, 17, 4, 19, 1, 10, 29, 22, 32, 16, 5, 3, 47, 18, 15, 1, 20, 9, 2, 28, 26, 21, 13, 31, 24, 15, 1, 4, 23, 2, 18, 46, 21, 17, 51, 14, 15, 1, 24, 19, 2, 8, 10, 1, 33, 27, 24, 25, 1, 20, 19, 12, 18, 30, 1, 23, 7, 14, 29, 6, 20, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Similar to A323832, but is different in that at each step, the "delete any run of identical digits" operation (A321801) is apply only once here, whereas in A323832, at each step after doubling the number, the operation A321801 is applied repeatedly until the number does not change any longer. The first term which differs from A323832 is a(66).

Conjecture: every number will eventually reach one of 0, 1, or 5.

Conjecture is true for n < 10^10.

2404877 takes 123 steps to reach 5 and is the largest value for a(n) for n < 10^7.

79620527 takes 131 steps to reach 5 and is the largest value for a(n) for n < 10^8.

769237841 takes 138 steps to reach 5 and is the largest value for a(n) for n < 10^9.

The numbers 4807736476 and 4807736509 both take 142 steps to reach 5 and this is the largest value for a(n) for n < 10^10.

LINKS

Chai Wah Wu, Table of n, a(n) for n = 0..10000

EXAMPLE

a(66) = 6: 66->132->264->528->1056->22->0. Contrast this with A323832(66) = 5: 66->132->264->528->1056->0.

PROG

(Python)

from re import split

def A306384(n):

    mset, m, c = set(), n, 0

    while True:

        if m == 1 or m == 0 or m == 5:

            return c

        m = int('0'+''.join(d for d in split('(0+)|(1+)|(2+)|(3+)|(4+)|(5+)|(6+)|(7+)|(8+)|(9+)', str(2*m)) if d != '' and d != None and len(d) == 1))

        if m in mset:

            return -1

        mset.add(m)

        c += 1

CROSSREFS

Cf. A321801, A321802, A323832.

Sequence in context: A040344 A261717 A323832 * A196188 A089294 A088934

Adjacent sequences:  A306381 A306382 A306383 * A306385 A306386 A306387

KEYWORD

nonn,base

AUTHOR

Chai Wah Wu, Feb 12 2019

STATUS

approved

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Last modified September 21 01:47 EDT 2019. Contains 327252 sequences. (Running on oeis4.)