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A264734 Prime powers k such that k - 2 and k + 2 are prime powers. 2
3, 5, 7, 9, 11, 25, 27, 29, 81, 241, 59051, 450283905890997361, 36472996377170786401 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

From Robert Israel, Nov 22 2015: (Start)

a(14) > 3^1000 - 2 if it exists.

One of a(n), a(n)+2 and a(n)-2 must be a power of 3. (End)

LINKS

Table of n, a(n) for n=1..13.

EXAMPLE

81 is in this sequence because 81 - 2 = 79, 81 and 81 + 2 = 83 are all prime powers.

MAPLE

ispp:= proc(x) local p, r;

  if isprime(x) then return true fi;

  p:= 2;

  do

     r:= iroot(x, p);

     if r^p = x then return isprime(r) fi;

     if r < 2 then return false fi;

     p:= nextprime(p);

  od:

end proc:

ispp(1):= true:

A:= NULL;

for n from 1 to 1000 do

  B:= map(ispp, [3^n-4, 3^n-2, 3^n+2, 3^n+4]);

  if B[1] and B[2] then A:= A, 3^n-2 fi;

  if B[2] and B[3] then A:= A, 3^n fi;

  if B[3] and B[4] then A:= A, 3^n+2 fi;

od:

A; # Robert Israel, Nov 22 2015

MATHEMATICA

Prepend[Select[Range@ 100000, AllTrue[{# - 2, #, # + 2}, PrimePowerQ] &], 3] (* Michael De Vlieger, Dec 03 2015, Version 10 *)

PROG

(MAGMA) [n: n in [5..100000] | IsPrimePower(n-2) and IsPrimePower(n) and IsPrimePower(n+2)];

(PARI) is(k) = isprimepower(k) || k==1;

for(k=1, 1e6, if(is(k) && is(k+2) && is(k-2), print1(k, ", "))) \\ Altug Alkan, Nov 22 2015

CROSSREFS

Cf. A000961, A144234, A264744.

Sequence in context: A088049 A229364 A029660 * A004156 A081936 A238795

Adjacent sequences:  A264731 A264732 A264733 * A264735 A264736 A264737

KEYWORD

nonn,more

AUTHOR

Juri-Stepan Gerasimov, Nov 22 2015

EXTENSIONS

a(12) and a(13) from Robert Israel, Nov 22 2015

STATUS

approved

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Last modified August 11 15:12 EDT 2020. Contains 336428 sequences. (Running on oeis4.)