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A124113 a(n) = largest composite number of n decimal digits that becomes prime by decreasing any one of the higher-order digits. 1
99, 987, 9951, 99939, 996983, 9981153, 99745289, 981196517, 9927756573, 99243774623, 992617385333, 9751651447377 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

COMMENTS

a(n) == 0 (mod 7) for n > 6.

What is the first n for which a(n) does not exist? Rough heuristics suggest before 20. - Charles R Greathouse IV, Nov 18 2010

LINKS

Table of n, a(n) for n=2..13.

EXAMPLE

a(5)=99939 is included as 99929, 998939, 98939 and 89939 are primes.

PROG

(PARI) A124113(d)={

    forstep(n=10^d-1, 10^d\9, [-2, -4, -2, -2],

        if(isA124113(n, d), return(n))

    )

};

isA124113(n, d)={

    if(n%3==1, return(0));

    for(i=1, d-1,

        if(!isprime(n-10^i), return(0))

    );

    my(v=eval(Vec(Str(n))));

    for(i=1, #v,

        if(v[i]==0, return(0))

    );

    !isprime(n)

};

CROSSREFS

Sequence in context: A177686 A135219 A182672 * A223157 A223155 A308466

Adjacent sequences:  A124110 A124111 A124112 * A124114 A124115 A124116

KEYWORD

hard,nonn,base

AUTHOR

Anton Vrba (antonvrba(AT)yahoo.com), Nov 29 2006

EXTENSIONS

a(2) corrected, a(7) inserted, and a(11)-a(12) added by Charles R Greathouse IV, Nov 15 2010

a(13) added by Charles R Greathouse IV, Nov 17 2010

STATUS

approved

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Last modified September 25 03:07 EDT 2022. Contains 356953 sequences. (Running on oeis4.)