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A245359
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Largest number k such that d_1^j + d_2^j + … + d_r^j is prime for all j = 1, 2, .. k, or 0 if no such k exists, where d_1, d_2, … d_r are the digits of n. a(n) = -1 if k is infinite.
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0
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0, 1, 1, 0, 1, 0, 1, 0, 0, 0, -1, 2, 0, 2, 0, 2, 0, 0, 0, 1, 2, 0, 2, 0, 2, 0, 0, 0, 1, 1, 0, 2, 0, 1, 0, 0, 0, 2, 0, 0, 2, 0, 1, 0, 0, 0, 1, 0, 2, 1, 0, 2, 0, 0, 0, 2, 0, 2, 0, 0, 2, 0, 0, 0, 2, 0, 1, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 2, 0, 2, 0, 0, 0, 1, 0, 0, 1, 0, 2
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OFFSET
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1,12
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COMMENTS
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If a(n) = K and reorder the digits of n to make a new number, n'. Thus, a(n') = K.
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LINKS
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FORMULA
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EXAMPLE
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1^1 + 2^1 = 3 is prime.
1^2 + 2^2 = 5 is prime.
1^3 + 2^3 = 9 is not prime.
So a(12) and a(21) = 2.
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PROG
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(PARI) a(n) = for(k=1, 10^3, d=digits(n); if(!ispseudoprime(sum(i=1, #d, d[i]^k)), return(k-1))); return(-1)
n=1; while(n<100, print1(a(n), ", "); n++)
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CROSSREFS
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KEYWORD
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sign,base
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AUTHOR
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STATUS
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approved
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