prog.sf

#!/usr/bin/ruby

# a(n) is the smallest Fermat pseudoprime to base 2 of the form p * (n*(p-1) + 1), where p is prime.

# The first few terms, are:
#   1194649, 2701, 341, 1387, 4681, 31609, 65281, 14491, 486737, 976873, 10261, 264773, 2757241, 48462301, 742813, 4369, 8534233, 181901, 65077, 489997, 220729, 80581, 107360641, 188057, 9564169, 79854409, 611701, 294271, 489994201, 61219789, 15709, 65301013, 132332201, 2909197, 215749, 123251, 3567481, 429135841, 64148717, 47220367, 2182802689, 257590661, 688213, 22087477, 15479777, 582289, 162690481, 81433591, 55109401, 52072021

func a(n) {
    each_prime(1e9, {|p|
        var r = (n * (p-1) + 1)
        if (r.is_prime && is_pseudoprime(p * r)) {
            return p*r
        }
    })
}

say 50.of { a(_+1) }

__END__
for k in (1..500) {
    say a(2**k)
}

__END__
for k in (1..500) {
    #say [k, a(k)]
    say a(k)
}