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A113029 a(1) = 2, a(2) = 3; for n>2, a(n) = least prime equal to the sum of two or more previous terms. 0
2, 3, 5, 7, 17, 19, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A heuristic argument suggests that all primes except 11, 13 and 23 are included in this series (tested on first million primes). - Ryan Murphy (murphy(AT)minegoboom.com), Jan 24 2006

Except for 17 which uses all 4 of the previous terms, all the other terms so far use only two or three of the previous terms. This is a more restrictive application of the Goldbach conjecture. - Robert G. Wilson v, Apr 08 2007, May 05 2007

Up to 10^4, all a(n) requiring 4 terms are of the form a(n)=2+7+m+p with m=5 or m=19, i.e., of the form 14+p or 28+p; no a(n)<10^6 requires more than 4 terms. - M. F. Hasler, May 04 2007

Ryan Murphy's heuristic is correct: a(n) = prime(n+3) for n > 6. It suffices to check that all n in 36..72 are the sum of one or more members of this sequence. Thus, all n > 72 are the sum of two or more distinct members of this sequence, since by Bertrand's postulate there is a prime n/2 < p < n. - Charles R Greathouse IV, Aug 22 2011

LINKS

Table of n, a(n) for n=1..58.

EXAMPLE

5 = 2+3 follows 3, 7 = 5+2 follows 5, 17 = 2+3+5+7 follows 7.

MATHEMATICA

(* first do *) Needs["DiscreteMath`Combinatorica`"] (* then *) f[lst_List] := Block[{k = Length@ lst, p = Infinity, q}, lmt = If[k > 5, Sum[Binomial[k, i], {i, 2, 4}], 2^k + 1]; k++; While[k < lmt, q = Plus @@ NthSubset[k, lst]; If[ ! MemberQ[lst, q] && PrimeQ@q && q < p, p = q]; k++ ]; Append[lst, p]]; Nest[f, {2, 3}, 58] (* Robert G. Wilson v *)

PROG

(PARI) prevprime(p)={ if( nextprime(p-1)<p || p<3, return((p-1)*(p>2))); p=bitor(p-3, 1); while( nextprime(p) > p, p-=2 ); p } \ decomp(n, p)={local(d); if(!p, if(n==2 || n==3, return([n]), p=n), p=min(n, p)); while( p=prevprime(p), if( bittest(disallowed, p), next); if( (n<2*p && isprime(n-p) && !bittest(disallowed, n-p) && d=[n-p]) || d=decomp( n-p, p ), return(concat(d, p)) ))} \ disallowed=0; forprime(p=1, 10^4, if(decomp(p), print1(p", "), disallowed+=1<<p)) \\ M. F. Hasler, May 04 2007

(PARI) a(n)=if(n>6, prime(n+3), [2, 3, 5, 7, 17, 19][n]) \\ Charles R Greathouse IV, Aug 22 2011

CROSSREFS

Sequence in context: A214588 A089968 A164060 * A090432 A127042 A069802

Adjacent sequences:  A113026 A113027 A113028 * A113030 A113031 A113032

KEYWORD

easy,nonn

AUTHOR

Amarnath Murthy, Jan 03 2006

EXTENSIONS

More terms from Ryan Murphy (murphy(AT)minegoboom.com), Jan 24 2006

Edited by N. J. A. Sloane at the suggestion of Andrew Plewe, May 14 2007

STATUS

approved

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Last modified February 23 11:22 EST 2018. Contains 299564 sequences. (Running on oeis4.)