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A222579 Least prime p_m with p_m+1 practical such that n=p_m -p_{m-1}+...+(-1)^{m-k}p_k for some 0<k<m with p_k-1 practical. 4
3, 5, 7, 5, 7, 11, 19, 11, 11, 17, 19, 17, 17, 23, 19, 23, 23, 31, 31, 41, 23, 41, 31, 47, 29, 47, 41, 59, 53, 59, 47, 59, 59, 79, 41, 83, 59, 79, 47, 83, 71, 83, 53, 83, 47, 103, 79, 107, 53, 103, 59, 103, 89, 103, 71, 131, 79, 127, 103, 131, 79, 127, 83, 149, 71, 127, 89, 127, 107, 127, 79, 191, 83, 149, 107, 197, 83, 149, 131, 167, 139, 149, 103, 149, 89, 149, 103, 167, 127, 179, 149, 167, 107, 167, 139, 167, 107, 179, 103, 179 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjecture: a(n)<=3n for all n>0. Moreover, a(2n-1)/(2n-1) and a(2n)/(2n) have limits 1 and 2 respectively, as n tends to the infinity.

LINKS

Zhi-Wei Sun, Table of n, a(n) for n = 1..10000

Zhi-Wei Sun, On functions taking only prime values, arXiv:1202.6589.

EXAMPLE

a(6)=11 since 6=11-7+5-3 with 12 and 2 both practical;

a(7)=19 since 7=19-17+13-11+7-5+3-2 with 20 and 1 both practical;

a(806)=p_{358}=2411 since 806=p_{358}-p_{357}+...+p_{150}-p_{149} with p_{358}+1=2412 and p_{149}-1=858 both practical. Note that a(806)/806 is about 2.9913.

MATHEMATICA

f[n_]:=f[n]=FactorInteger[n]

Pow[n_, i_]:=Pow[n, i]=Part[Part[f[n], i], 1]^(Part[Part[f[n], i], 2])

Con[n_]:=Con[n]=Sum[If[Part[Part[f[n], s+1], 1]<=DivisorSigma[1, Product[Pow[n, i], {i, 1, s}]]+1, 0, 1], {s, 1, Length[f[n]]-1}]

pr[n_]:=pr[n]=n>0&&(n<3||Mod[n, 2]+Con[n]==0)

pp[k_]:=pp[k]=pr[Prime[k]+1]==True

pq[k_]:=pq[k]=pr[Prime[k]-1]==True

s[0_]:=0

s[n_]:=s[n]=Prime[n]-s[n-1]

Do[Do[If[pp[j]==True&&pq[i+1]==True&&s[j]-(-1)^(j-i)*s[i]==m, Print[m, " ", Prime[j]]; Goto[aa]], {j, PrimePi[m]+1, PrimePi[3m]}, {i, 0, j-2}];

Print[m, " ", counterexample]; Label[aa]; Continue, {m, 1, 100}]

CROSSREFS

Cf. A000040, A005153, A210479.

Sequence in context: A225889 A070647 A070949 * A141574 A141261 A077129

Adjacent sequences:  A222576 A222577 A222578 * A222580 A222581 A222582

KEYWORD

nonn

AUTHOR

Zhi-Wei Sun, Feb 25 2013

STATUS

approved

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Last modified May 25 11:07 EDT 2020. Contains 334592 sequences. (Running on oeis4.)