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a(A078129(n))=0; a(A078130(n))=1; a(A078131(n))>0;

Conjecture (lower bound): for all k exists b(k) such that a(n)>k for n>b(k); see b(0)=A078129(83)=154 and b(1)=A078130(63)=218.

From Hieronymus Fischer, Nov 11 2007: (Start)

Equivalently, prime numbers which cannot be written as sum of squares of primes (see A078137 for the proof).

Equivalently, prime numbers which cannot be written as sum of squares of 2 and 3 (see A078137 for the proof).

The sequence is finite, since numbers > 23 can be written as sums of squares >1 (see A078135).

Explicit representation as sum of squares of primes, or rather of squares of 2 and 3, for numbers m>23: we have m=c*2^2+d*3^2, where c:=(floor(m/4) - 2*(m mod 4))>=0, d:=m mod 4. For that, the finiteness of the sequence is proved. (End)

A078128(a(n))=0.

The sequence is finite because every number greater than 181 can be represented using just 8 and 27. - Franklin T. Adams-Watters, Apr 21 2006

More generally, the numbers which are not the sum of k-th powers larger than 1 are exactly those in [1, 6^k - 3^k - 2^k] but not of the form 2^k*a + 3^k*b + 5^k*c with a,b,c nonnegative. This relies on the following fact applied to m=2^k and n=3^k: if m and n are relatively prime, then the largest number which is not a linear combination of m and n with positive integer coefficients is mn - m - n. - Benoit Jubin, Jun 29 2010

Equivalent to primes which can be written as the sum of cubes of primes; the question being "what is the minimum number of terms in such sums when they can be written in more than one way? - Jonathan Vos Post, Sep 21 2006

Mikawa and Peneva: "One of the famous and still unsettled problems in additive prime number theory is the conjecture that every sufficiently large integer satisfying some natural congruence conditions, can be written as the sum of four cubes of primes. Although the present methods lack the power to prove such a strong result, Hua... has been able to prove that every sufficiently large odd integer as the sum of nine cubes of primes. He also established that almost all integers {n == 1 mod 2, n =/= 0, +/-2 mod 9, n =/= 0 mod 7} can be expressed as the sum of five cubes of primes." - Jonathan Vos Post, Sep 21 2006