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 A344481 Isolated single primes enclosed by four composites on square spiral board of odd numbers. 1
 97, 157, 233, 257, 293, 307, 331, 337, 359, 367, 389, 397, 409, 439, 449, 479, 487, 499, 503, 563, 607, 613, 631, 653, 677, 683, 691, 709, 727, 743, 751, 761, 773, 853, 863, 887, 907, 911, 929, 937, 967, 971, 983, 1013, 1069, 1087, 1117, 1181, 1187, 1193, 1201 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS EXAMPLE 3 is not a term because two of the four neighbors (1, 5, 17 and 21) are primes. 97 is a term because 97 is a prime and all four neighbors (51, 95, 99 and 159) are composites (see the illustration in Links). PROG (Python) from sympy import prime, isprime; from math import sqrt, ceil def neib(m):     n = int(ceil((sqrt(m)+1.0)/2.0)); L = [m, m, m, m]     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     L[0]+=1 if m

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Last modified September 24 22:33 EDT 2021. Contains 347651 sequences. (Running on oeis4.)