1,1

n in base 1 is understood to be the concatenation of n 1's. Hence a prime in base 1 has a prime number of digits. The (first) seven terms listed also have a prime number of digits when expressed in any negative base b with -p<= b <= -2. Hence they have a noncomposite number of digits for any meaningful base b (for the common use of "base") as only one digit is required whenever b > p or b < -p.

Once you reach the 169th prime, which is 1009, at least for base 10, its representation is of composite length. For any prime above this, at some point in the base representations you must have a base representation that is only four in length.

7 is a term because 7 = 1111111 (base 1) = 111 (base 2) = 21 (base 3) = 13 (base 4) = 12 (base 5) = 11 (base6) = 10 (base 7) and the number of digits is 7, 3, or 2, a prime, for each base 1 <= b <=7. Also, in support of the comments, 7 = 11011 (base -2) = 111 (base -3) = 133 (base -4) = 142 (base -5) = 151 (base -6) = 160 (base -7), 3 or 5 digits, again a prime length, for each negative base with -7 <= b = -2.

Do[p = Prime[n]; k = 2; While[k < p && PrimeQ[ Length[ IntegerDigits[p, k]]], k++ ]; If[k == p, Print[p]], {n, 1, 10^6}]

Sequence in context: A059471 A059496 A066814 this_sequence A042994 A129692 A042995

Adjacent sequences: A072882 A072883 A072884 this_sequence A072886 A072887 A072888

base,nonn,full,fini

Rick L. Shepherd (rshepherd2(AT)hotmail.com), Jul 28 2002

Edited by Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 01 2002