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A232657 Numbers producing at least 4 primes by proper concatenation of decrements. 2
100, 874, 1000, 1286, 3040, 3721, 3805, 3922, 4468, 6412, 7014, 7852, 8338 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The very simplest example of the type of prime concerned is 43, but 100 (see Example) is the first value to produce 4 primes. 1000 is the first to do so in the fastest way possible. Proper concatenation first excludes 73 from this list, as the stem--initial--value in concatenation is itself excluded from consideration.

8338 is the first value to produce 5 primes, with terminal values 8323, 8293, 8079, 7561 and 6519.

LINKS

Table of n, a(n) for n=1..13.

EXAMPLE

100 yields four primes in this way: 100 99, 100 99 98 97, 100 99 98 97 96 95 94 93 92 91, and 100 99 98 97...62 61.  As the first value to do so properly, a(1)=100.

PROG

(PARI) is(n)=my(t=Str(n), s=4); while(n--, t=Str(t, n); if(isprime(eval(t)) && s--==0, return(1))); 0 \\ Charles R Greathouse IV, Feb 18 2014

CROSSREFS

Cf. A152396, A152397, A172257.

Sequence in context: A233635 A233628 A030024 * A287027 A326638 A284515

Adjacent sequences:  A232654 A232655 A232656 * A232658 A232659 A232660

KEYWORD

nonn,base,more

AUTHOR

James G. Merickel, Nov 27 2013

STATUS

approved

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Last modified February 26 11:15 EST 2021. Contains 341631 sequences. (Running on oeis4.)