A359447 a(n) is the least number that is the sum of two cubes of primes and is 2^n times an odd prime, or -1 if there is no such number.
-1, -1, 152, 2224, 9056, 108736, -1, 4532992, 34674176, 268684288, 2280249344, 18693763072, 138890141696, 1111848828928, 8803419521024, 70375767212032, 564861779443712, 4507018424221696, 36030079546425344, 288238419152207872, 2305850719072157696, 18446757709572210688, 147573952867129622528
Offset: 1
Keywords
Examples
a(3) = 152 because 3^3 + 5^3 = 152 = 2^3 * 19, 3 and 5 are primes and 19 is odd, and no smaller number works.
Programs
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Maple
f:= proc(n) local p,q,t; t:= 2^n; p:= nextprime(t/2); while p > 2 do p:= prevprime(p); q:= t - p; if isprime(q) and isprime(p^2 - p*q + q^2) then return p^3 + q^3 fi od; -1 end proc: map(f, [$1..20]);
Comments