OFFSET
1,1
COMMENTS
This sequence is similar to A172294, in which the starting number of the square spiral is 0 instead of 1. For a(n) < 10000000, 4 out of the 782 terms in this sequence, 72, 10302, 415380 and 1624350 are absent in A172294, while 6 out of the 784 terms in A172294, 42, 23562, 83232, 205662, 5805690 and 7019850 are absent in this sequence.
Conjecture: This sequence is infinite. If the conjecture holds, then the twin prime conjecture is true.
The 4 neighbors of n in the spiral are A068225, A068226, A334751, and A334752. - Kevin Ryde, Feb 13 2021
PROG
(Python)
from sympy import isprime
from math import sqrt, ceil
m, m_max = 2, 1000000
while m <= m_max:
L = [0, 0, 0, 0]
n = int(ceil((sqrt(m) + 1.0)/2.0))
z1 = 4*n*n - 12*n + 10
z2 = 4*n*n - 10*n + 7
z3 = 4*n*n - 8*n + 5
z4 = 4*n*n - 6*n + 3
z5 = 4*n*n - 4*n + 1
if m > z1 and m < z2: L = [m + 1, m - 8*n + 15, m - 1, m + 8*n - 7]
elif m > z2 and m < z3: L = [m + 8*n - 5, m + 1, m - 8*n + 13, m - 1]
elif m > z3 and m < z4: L = [m - 1, m + 8*n - 3, m + 1, m - 8*n + 11]
elif m > z4 and m < z5: L = [m - 8*n + 9, m - 1, m + 8*n - 1, m + 1]
if isprime(L[0]) == 1 and isprime(L[1]) == 1 and isprime(L[2]) == 1 and isprime(L[3]) == 1: print(m)
m += 2
CROSSREFS
KEYWORD
nonn
AUTHOR
Ya-Ping Lu, Feb 13 2021
STATUS
approved